departure vs driftage

departure and driftage both are nouns.

Word	Noun	Adjective	Verb	Adverb
departure	Yes	No	No	No
driftage	Yes	No	No	No

As nouns, driftage is a hyponym of departure; that is, driftage is a word with a more specific, narrower meaning than departure:

departure: a variation that deviates from the standard or norm
driftage: the deviation (by a vessel or aircraft) from its intended course due to drifting

Other hyponyms of departure include discrepancy, variance, variant, flection, flexion, inflection.

departure (noun)	driftage (noun)
the act of departing	the deviation (by a vessel or aircraft) from its intended course due to drifting
euphemistic expressions for death
a variation that deviates from the standard or norm