The shift action is a move action that allows a character to move 1 square without provoking opportunity attacks.

Moving out of a square adjacent to an enemy normally provokes opportunity attacks, but shifting does not do so.

Shifting is not possible when using a form of movement dependent on a skill check, for example, climbing or swimming, for most characters.

As usual, entering difficult terrain costs 1 extra square of movement. Since the shift action only allows 1 square of movement, entering difficult terrain is not possible with a shift action, unless some effect is allowing a character to shift multiple squares.