1540s, in geometry, of lines, "lying in the same plane but never meeting in either direction;" of planes, "never meeting, however far extended;" from Middle French parallèle (16c.) and directly from Latin parallelus, from Greek parallēlos "parallel," from para allēlois "beside one another," from para- "beside" (see para- (1)) + allēlois "each other," from allos "other" (from PIE root *al- "beyond"). Figurative sense of "having the same direction, tendency, or course" is from c. 1600.
As a noun from 1550s, "a line parallel to another line." Meanings "a comparison made by placing things side by side" and "thing equal to or resembling another in all particulars" are from 1590s. Parallel bars as gymnastics apparatus is recorded from 1868.