reciprocal (adj.)

"moving backwards and forwards," 1690s, present-participle adjective from reciprocate (v.). Specifically of machines, "having reciprocating parts," by 1822.

Reciprocating engine. A form of engine in which the piston and piston-rod move back and forth in a straight line, absolutely relative to the cylinder, as in oscillating-cylinder engines: in contradistinction to rotary engine. [Century Dictionary, 1895]

1610s, "to give and return mutually," a back-formation from reciprocation, or else from Latin reciprocatus, past participle of reciprocare "rise and fall, move back and forth; reverse the motion of," from reciprocus "returning the same way, alternating" (see reciprocal). Sense of "cause to move back and forth" is from 1650s; intransitive sense of "move backward and forward" is from 1670s. Meaning "to give or do in response, act in return or response" is from 1820. Related: Reciprocated; reciprocating.