There's been some recent debate over delineating what's "Celt" vs. what's "Scythian", discussed here:
Peter S. Wells, Beyond Celts, Germans, and Scythians: Archaeology and Identity in Iron Age Europe
For myself, I try to base my usage of these terms on how they were originally used by classical authors (what I'll call the linguistic definition of the Celts and Scythians), and to interpret archaeological finds (what I'll call the archaeological definition) in relation to that usage. Linguistically speaking, Herodotus introduced subsequent authors to the names "Celt" (History 2.33: "For the river Ister begins in Celtic country and the city of Pyrene cleaves Europe in two. The Celts dwell beyond the Pillars of Heracles, and they have common borders with the Cynesii, who live furthest of all people that inhabit Europe toward the west") and "Scythian" (History 1.15, etc.--see esp. the beginning of Book 4 where Herodotous discusses various ideas about Scythian origins). Since Herodotus other references to the Scythians have been found in Assyrian writings. My approach to defining the Scythians is to start with linguistic definitions based on the descriptions of Herodotus and the Assyrians, and to identify cultural characteristics and generate archaeological definitions on that basis. I take a similar approach to defining the Celts, starting from the descriptions of Herodotus, Caesar, Livy, etc. Applying this approach tends to define the Celts and Scythians as distinct groups living in distinct geographic areas during specific time periods as described by Herodotus and others.
A different approach is to define these groups by proceeding from archaeological data--so for instance, defining the Celts in terms of archaeological interpretations of finds at Halstatt and La Tene. This approach tends to define the Celts and Scythians less distinctly (cf. for example Celts and Scythians Linked by Archaeological Discoveries).
Based on the above considerations, I think it may be helpful in this type of discussion to distinguish between the Celts and Scythians as linguistically-defined groups and as archaeologically-defined groups, and when using the archaeological definition, to distinguish common ancestors/descendants of these groups from the groups themselves as defined by the archaeological sites/strata they are associated with. The term "proto-Celt" I believe blam used in one post is useful here. We might also speak of "proto-Scythians", as well as descendants of the Celts and/or Scythians--I don't know what we'd call those; I don't like "post-Celts" or "post-Scythians", but something that expresses that idea, maybe.