Cumulate and its far more common relative accumulate both come from the Latin word cumulare, meaning "to heap up." Cumulare, in turn, comes from cumulus, meaning "mass." (Cumulus functions as an English word in its own right as well. It can mean "heap" or "accumulation," or it can refer to a kind of dense puffy cloud with a flat base and rounded outlines.) Cumulate and accumulate overlap in meaning, but you're likely to find cumulatemostly in technical contexts. The word's related adjective, cumulative, however, is used more widely.