While that definition is in the dictionary, it is not the one scientists use in this instance. I would guess you are selecting that definition because you want to discredit the theory of evolution--"Its only a theory [guess]."
Please examine these definitions again:
Definitions (from a google search, with additions from this thread):
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." Addendum: "Theories do not grow up to be laws. Theories explain laws." (Courtesy of VadeRetro.)
Theory: A scientifically testable general principle or body of principles offered to explain observed phenomena. In scientific usage, a theory is distinct from a hypothesis (or conjecture) that is proposed to explain previously observed phenomena. For a hypothesis to rise to the level of theory, it must predict the existence of new phenomena that are subsequently observed. A theory can be overturned if new phenomena are observed that directly contradict the theory. [Source]
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. It is the dependence on verifiable data and the capability of testing that distinguish scientific theories from matters of faith.
Guess: an opinion or estimate based on incomplete evidence, or on little or no information.
I do realize that scientists have their very own dictionary.
Pretty convenient of them, huh?
Dear Sun: Beware evos who demand to set the terms, for they will control the debate if you allow them. And control is all these Asberger's sufferers (see wikipedia if you are unfamiliar with Asberger's Syndrome) are after. It's not like they'll have a date this Saturday night.
Dear Coyoteman--came across a nice little quote, and I thought of you--
Anthropologists believe that the plural of anecdote is “data.”
IOW--Anthropology is just sociology with relics. A social science trying to pass itself off as important.