reciprocate

Short definition from Wiktionary

Verb

1. 1
   To exchange two things, with both parties giving one thing and taking another thing.
2. 2
   To give something else in response (where the "thing" may also be abstract, a feeling or action) To make a reciprocal gift.
3. 3
   To move backwards and forwards, like a piston.
4. 4
   To counter, retort or retaliate.