The ceiling is the top of the visible portion of the playfield.

In some tetris games the ceiling can prevent a piece from rotating. It's common that these games lack wall kicks as well. Classic games with these rules include Tetris (IBM PC) and Tetris (Sega).

L cannot rotate

It falls a row

Now it can rotate freely

Modern games do not have this restriction.

Free to rotate

In Tengen Tetris, The New Tetris, and most SRS games, the pieces actually spawn above the ceiling.