A double check, as the name suggests, is a check delivered by two pieces at the same time. While not technically a tactic (as it is not a sequence of moves), a double check can play important roles in a sequence due its very forcing nature: the only legal moves are king moves.
A beautiful application of double checks often occurs in smothered mates:
1. Nh6+ is a double check (as both the knight and the queen deliver check
simultaneously). and it is the start of a beautiful sequence because
the only possible response.