Lock delay refers to how many frames a tetromino waits while on the ground before locking. Classic games lock tetrominoes immediately once it has fallen to the ground, while some newer games give the tetromino some time before locking. TGM2's T.A. Death mode infamously sets its lock delays as low as fifteen frames.
Some games such as the Puyo series use a fixed amount of lock delay for each piece, pausing the timer while the piece falls, resetting it only when a new piece enters (entry reset). Some games, especially those using Sega rotation or its successors, reset the timer every time the piece "steps" downward (step reset). Games following the Tetris Guideline generally reset the timer on any successful shifting or rotation (Infinity or move reset).