condense (v.)

early 15c., "thicken, make more dense or compact" (implied in condensed), from Old French condenser (14c.) or directly from Latin condensare "to make dense," from assimilated form of com-, here probably an intensive prefix (see com-), + densare "make thick," from densus "dense, thick, crowded," a word used of crowds, darkness, clouds, etc. (see dense).

Sense in chemistry and physics, "to reduce to another and denser form" (as a gas or vapor to a liquid) is from 1660s. Intransitive sense "become denser" is from 1650s. Related: Condensed; condensing.