The word Buddha is derived from the verb 'budh' which means 'to know'...at least according to the 16 volume oxford dictionary.
Knowing as opposed to ignorance...so the word Buddha really means 'the knower'.
A few historical Zen masters I used to read about claimed that no monk is truly a Buddhist until they attain some measure of enlightenment.
that Sanskrit root (budh) also appears in numerous vedic texts too and is explained in the same way (knowledge which gives enlightenment etc)
Obviously to be a "buddhist" suggests more of a methodology than a linguistic investigation of "budh" suggests