The term "Celt" is often associated with the cultures of Hallstatt and La Tène, but what makes you think that the Central European Bronze-Age cultures from 2000 to 1200 BCE (Unetice, Tumulus, Urnfield) were not related and also (Proto-)Celtic speaking ? If there is a genealogical, cultural and linguistic continuity, we are talking about the same people. It might be more correct to call them Proto-Celts, but that's really just a fine nuance.
I associate the term celt with Hallstatt and La Tène. We need a starting point, i don't say that there is no relation between the central european Bronze-Age and the future Hallstatt and La Tène celts, but with that logic we can go deep in the past and the celtic notion lose all it's sense.