Line clear

The object of Tetris is to last as long as possible before the screen fills up with tetrominoes. To do this, you must assemble the tetrominoes to form one or more rows of blocks that span the entire playing field, called a line clear. When you do so, the row will disappear, causing the ones above it to settle.

Single
A single is the act of clearing one line at a time:

Double
A double is when two lines are cleared at once:

Triple
A triple is three lines cleared simultaneously. Most games allow only I, L, and J tetrominoes to complete a triple, but newer games with SRS allow S, Z, and T tetrominoes to twist into seemingly impossible positions.

Tetris
A tetris is four lines cleared simultaneously. In most games, this can only be done with an "I" tetromino.

Hurdle
In Tetris DS mission mode, the act of clearing two or three lines separated by one or more rows with gaps is called a hurdle. Any tetromino that can complete a triple can complete a hurdle:

Back-to-Back
Back-to-Back clears are any combination of two or more T-Spin Single, T-Spin Double, T-Spin triple, or Tetris, in the case of the recent games Tetris Worlds and Tetris DS. In certain Tetris game modes, you can earn extra points or deal an additional line of garbage by clearing Back-to-Back.

For example, a player can clear a line with a T-Spin Single using the T tetromino, then clear four lines with an I tetromino, still earning him a Back-to-Back tetris.

Naive
In naive gravity, the blocks above a cleared line move down by exactly the number of cleared lines below them. This can often leave floating blocks, unconnected to anything, after a line is cleared. Most tetromino based games use naive gravity.

Sticky
The playfield is divided into connected segments using flood fill. Any blocks that are adjacent horizontally or vertically are marked as one segment. Each segment falls independently until it meets the floor or another block. Additional line clears may result.

Games that use sticky gravity:
 * Super Scope 6 Blastris A
 * Tetris Blast and Bombliss
 * The Next Tetris (modifies rule such that only blocks of a single color can form a segment)
 * Re-released as Tetris Worlds Sticky
 * Lockjaw: The Overdose

Cascade

 * Tetris 2 (Tetris Flash)
 * Quadra
 * Tetris Worlds Cascade and Fusion
 * Tetris DS Touch