Double attack
A move that creates two threats at once, more than the opponent can answer in a single reply.
The double attack is the most general tactical idea in chess, and many of the named motifs — forks, discovered attacks, batteries — are special cases of it. A single move generates two threats, and the opponent cannot address both. The attacker collects whichever target the defender abandons.
The fork is the most concrete double attack: one piece attacks two targets at once. The discovered attack is a double attack with two different attackers working together. Even a quiet positional move can be a double attack — a pawn advance that threatens a knight while also opening a file for a rook is a double attack in everything but name.
The classical pattern in tactical training is the queen-fork double attack: the queen moves to a square where it both threatens mate (or a piece) and attacks an undefended piece on another part of the board. The opponent saves against the bigger threat; the queen takes the smaller one. This pattern appears in thousands of games and is the most common way to gain material in quiet positions.
The defence against double attacks is anticipation. Once both threats are on the board, the position is usually lost. Strong players spend most of their calculation looking for moves that contain hidden double attacks two or three plies away.