Which mission achieved the first rendezvous between two crewed spacecraft in orbit?

Gemini 6A and Gemini 7 achieved the first rendezvous between two crewed spacecraft in orbit.

During the December 1965 mission, Gemini 6A, crewed by Walter Schirra and Thomas Stafford, approached Gemini 7, crewed by Frank Borman and James Lovell. The spacecraft flew as close as about 30 centimetres and maintained formation for several hours.

The achievement demonstrated that astronauts could navigate between spacecraft in orbit, a capability essential to later lunar missions. Gemini 6A launched on 15 December 1965, after an earlier launch attempt was aborted, and rendezvoused with Gemini 7 the same day.

Rendezvous means matching orbits and approaching another spacecraft; docking is a separate step. Gemini 8 later achieved the first docking between two spacecraft, so the two milestones should not be confused.