accelerant n.

1520s, "hasten the occurrence of;" 1590s, "make quicker" (implied in accelerating), from Latin acceleratus, past participle of accelerare "to hasten, quicken" (trans.), "make haste" (intrans.), from ad "to" (see ad-) + celerare "hasten," from celer "swift," which is perhaps from PIE *keli- "speeding" (see celerity). The intransitive sense of "go faster, become faster" in English is from 1640s. Related: Accelerated; accelerative.