vi. & vt.

1. 交換；報答；互給
2. 【機】（使）往復運動

v.

1. respond to (a gesture or action) with a corresponding one.
   ▸return (love, affection, etc.) to someone who gives it.
2. [usu. as adj. reciprocating] (of a machine part) move backwards and forwards in a straight line.