The ceiling is the top of the visible portion of the playfield.
Not all games have the pieces interact with the ceiling the same way.
Enter above ceiling
In most newer games and a few older games, pieces enter the playfield at least partially in the vanish zone.
Enter below ceiling
The majority of games produced prior to 2001 placed new pieces with their top edge touching the ceiling.
This also includes Ti (2005), as one of the several Guideline exemptions that it received for consistency with TAP.
Enter below solid ceiling
In some 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.