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DRAFT THREE:

These are still mostly Coyoteman's definitions, again in slightly different order (not yet satisfactory), with my contributions and those of others indented and identified by contributor:

Hypothesis: a tentative theory about the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena; "a scientific hypothesis that survives experimental testing becomes a scientific theory"; "he proposed a fresh theory of alkalis that later was accepted in chemical practices"

Speculation: a hypothesis that has been formed by speculating or conjecturing (usually with little hard evidence)

By balrog666 (post 8): Off the top of my head, I would say that all scientists speculate to some degree and many discuss, and some even publish, such thoughts (particularly to get some quick and easy feedback). However, such speculations are not indicative of beliefs held, not intended to indicate a direction for future research, but to simply to explore an idea, no matter how off-the-wall it may be, and see if any fruitful insights are realized. OTOH, some of it is intended to be read with tongue firmly in cheek.
By furball4paws in post #15: A scientific speculation is much different than any old speculation. When a scientist speculates he is drawing on experience, patterns and somewhat unrelated things that are known or appear to be unlikely. This becomes a very informed guess. The better the scientist and the greater his experience, the better chance his speculations will prove to be true.

Guess: an opinion or estimate based on incomplete evidence, or on little or no information

Law: a generalization that describes recurring facts or events in nature; "the laws of thermodynamics"

Assumption: premise: a statement that is assumed to be true and from which a conclusion can be drawn; "on the assumption that he has been injured we can infer that he will not to play"

Model: a simplified framework designed to illuminate complex processes; a hypothetical description of a complex entity or process; a physical or mathematical representation of a process that can be used to predict some aspect of the process

By spunkets (post #24):
I'd also suggest that this be changed to: Model: a simplified ~~framework~~ representation designed to illuminate complex processes; a hypothetical description of a complex entity or process; a physical or mathematical representation of a process that can be used to predict some aspect of the process . The reason is that all models are representations of an object, or process. "Framework" doesn't fit.
You might also want to add the correspondance principle. The Correspondance Principle says, "Any new theory must contain the old theory as a limiting case." The reason for the incluison is that some folks think that theories can be replaced and the old one junked. That's never true. The old theory must always appear as a limiting case.

Observation: any information collected with the senses

Data: factual information, especially information organized for analysis or used to reason or make decisions

Fact: when an observation is confirmed repeatedly and by many independent and competent observers, it can become a fact

Belief: any cognitive content (perception) held as true; religious faith

By spunkets (post #23):
The word belief is generally used with some corresponding Bayesian probability of being true. That depends on the priors(cognitive content) of that individual's cognitive system and whether, or not that cognition is rational, or irrational. The distinguishing feature is rationality, or the absence of it, in the belief system.
Knowledge in a similar way depends on the belief system. One can have knowledge of bagvita's belief system, of which bagvita's beliefs were his knowledge. Both the words belief and knowledge require qualifiers. In general though, the word knowledge implies that one actually knows.

By PH:
Strictly speaking, what one "believes" on faith and what one "knows" are different things.
Belief: One can "believe" in the existence of the tooth fairy, but one does not -- in the same sense of the word -- "believe" in the existence of his own mother.
Faith: Belief in the first proposition (tooth fairy) requires faith, which is belief in something for which there is no evidence or logical proof.
Knowledge: The second proposition (mother) is the kind of knowledge which follows from sensory evidence.
Logical Proof: There is also that kind of knowledge (like the Pythagorean theorem) which follows from logical proof. In either case -- that is, knowledge acquired from sensory evidence or demonstrated by logical proof -- there is no need for faith, and that term is inapplicable to such knowledge.
Proof: Except for math and geometry, there is little that is actually proven. Even well-established scientific theories can't be conclusively proven, because -- at least in principle -- a counter-example might be discovered. Scientific theories are always accepted provisionally, and are regarded as reliable only because they are supported (not proven) by the verifiable facts they purport to explain and by the predictions which they successfully make. All scientific theories are subject to revision (or even rejection) if new data are discovered which necessitates this.

Faith: the belief in something for which there is no evidence or logical proof; acceptance of ideals, beliefs, etc., which are not necessarily demonstrable through experimentation or reason. [By PH: This is similar to what I said, above.]

By <1/1,000,000th% (post #4): The belief in something for which there is no material evidence or empirical proof; acceptance of ideals, beliefs, etc., which are not necessarily demonstrable through experimentation or observation.

Theory: a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena; "theories can incorporate facts and laws and tested hypotheses." [That is Webster's definition #1, as pointed out by Quark2005 in post #9.] Addendum: "Theories do not grow up to be laws. Theories explain laws." (Courtesy of VadeRetro.)

By PH
Scientific theory: In between mother (knowledge from sensory evidence) and the Pythagorean theorem (knowledge from logical proof) are those propositions we provisionally accept (or in common usage "believe"), like relativity and evolution, because they are currently successful scientific theories -- testable, and therefore falsifiable explanations of the available, verifiable data (which data is knowledge obtained via sensory evidence). Here too, there is no need for faith, and that term does not apply in the context of scientific theories.
Confidence: When a scientific theory has a long history of being supported by verifiable evidence, it is appropriate to speak about "acceptance" of (not "belief" in) the theory; or we can say that we have "confidence" (not "faith") in the theory. The word "faith" is inapplicable in this context.
It is the dependence on verifiable data and the capability of testing that distinguish scientific theories from matters of faith. Purely theological matters that are believed on faith are not capable of being tested, and thus theological doctrines are not scientific.

Dogma: a religious doctrine that is proclaimed as true without proof

Religion:

theistic: "1 the belief in a superhuman controlling power, esp. in a personal God or gods entitled to obedience and worship. 2 the expression of this in worship. 3 a particular system of faith and worship."
Non-Theistic: "The word religion has many definitions, all of which can embrace sacred lore and wisdom and knowledge of God or gods, souls and spirits. Religion deals with the spirit in relation to itself, the universe and other life. Essentially, religion is belief in spiritual beings. As it relates to the world, religion is a system of beliefs and practices by means of which a group of people struggles with the ultimate problems of human life."

Impression: a vague idea in which some confidence is placed; "his impression of her was favorable"; "what are your feelings about the crisis?"; "it strengthened my belief in his sincerity"; "I had a feeling that she was lying" [Observation by PH: should be mentioned, perhaps, that this is subjective.]

Opinion: a personal belief or judgment that is not founded on proof or certainty.

What follows are the rest of PH's offerings, and some others, not yet integrated into the above:

Reason: "Reason -- the faculty which identifies and integrates the material provided by man's senses -- is man's only means of perceiving reality, his only source of knowledge, his only guide to action, and his basic means of survival." -- Ayn Rand

Distinctions between reason and faith: It is necessary to distinguish between an axiom (which is a logical necessity) and an article of dogma (an arbitrary assumption), between objective fact and subjective experience, and between hypothesis (a proposed, testable explanation of an observed phenomenon) and conjecture (a guess based on virtually no data). Understanding these fundamental concepts allows us to distinguish reason-based science from faith-based doctrine. Reason and faith are commonly confused, but they are very different intellectual enterprises, with different goals. When properly understood, they are not in conflict.

Occam's Razor. [Suggested by Virginia-American (post #30).

Karl Popper's Falsification. [Suggested by RA's post #36. Possibly: ] A theory can never be proved right by agreement with observation, but it can be proved wrong by disagreement with observation. Because of this asymmetry, science makes progress uniquely by proving that good ideas are wrong so that they can be replaced by even better ideas. ... Almost without exception, in order to extract a falsifiable prediction from a theory, it is necessary to make additional assumptions beyond the theory itself. Then, when the prediction turns out to be false, it may well be one of the other assumptions, rather than the theory itself, that is false. To take a simple example, early in the twentieth century it was found that the orbits of the outermost planets did not quite obey the predictions of Newton’s laws of gravity and mechanics. Rather than take this to be a falsification of Newton’s laws, astronomers concluded the orbits were being perturbed by an additional unseen body out there. They were right. That is precisely how the planet Pluto was discovered.