The launch of two threats simultaneously. It is different from a fork in that either or both threats need not be a capture.
An attack against two enemy pieces at the same time. If a single piece is attacking two enemy pieces, it is a fork.
An attack on two (or more) pieces by a single move
An attack against two pieces or Pawns at the same time.
Two attacks made with one move: these attacks may be made by the same piece (in which case it is a '' fork''); or by different pieces (a situation which may arise via a discovered attack in which the moved piece also makes a threat). The attacks may directly threaten opposing pieces, or may be threats of another kind: for instance, to capture the queen and deliver checkmate.
to attack two pieces at the same time with one move; 1.e4 attacks d5 and f5. Chernev and Reinfeld say that this is one of the two central themes of tactical play (the other being concentration of force). Lots of examples in the Tactics section of the Canon.