Shift

Moving through a fierce battle is dangerous; a creature must be careful to avoid a misstep that gives a foe a chance to strike a telling blow. The way to move safely when enemies are nearby is to shift. When a creature wants to flee, the safest way to do that is to first shift away from an adjacent enemy, then walk or run.

Shift
         Action: Move action.
         Movement: The creature moves 1 square. (Some powers and effects allow creatures to move farther than 1 square with a shift.)
         No Opportunity Actions Triggered: Unless the description of an effect says otherwise, shifting doesn’t trigger opportunity actions such as opportunity attacks.
         Special Movement Modes: A creature can’t shift when using a movement mode that requires it to make a skill check. For instance, if it is climbing, it can’t shift if it has to make an Athletics check to climb. (A creature that has a climb speed can shift while climbing.)

Because a square of difficult terrain costs 1 extra square of movement to enter, a creature can’t normally shift into such a square, unless it is able to shift multiple squares or ignores the effects of difficult terrain.

Published in Player's Handbook, page(s) 292, Rules Compendium, page(s) 249.