The phase between the opening and the endgame.
The middle game typically starts when one part of the board is heavily contested by both players and a fight occurs. The fight will mostly move out into the center. Play in the middle game is dominated by attack and defence.
When the situation calms down and territory becomes again the major factor, the endgame has started. The middle game therefore typically ends when all important groups are secured against killing attacks.
Basic concepts for understanding the middle game are weak groups, frameworks, and thickness. Basic patterns in the middle game are sometimes called middlegame joseki, eg the crosscut, capping play, and shoulder hit. These patterns have to be applied in the presence of existing positions on the board, and are therefore more difficult to master than the corner openings.