\Pos"si*bly\, adv. In a possible manner; by possible means; especially, by extreme, remote, or improbable intervention, change, or exercise of power; by a chance; perhaps; as, possibly he may recover.
\Prob"a*bly\, adv. In a probable manner; in likelihood.
Distinguish between what may possibly and what will probably be done. --L'Estrange.
\Def"i*nite*ly\, adv. Without question and beyond doubt; in a definite manner; with precision; precisely; determinately.
Em, these three words are *not* interchangeable.
And don't you forget it.