Peep
A peep is a forcing move to which opponent has just a single answer to defend against an immediate threat. This situation occurs if one threatens to cut a connection between major groups or key stones of the opponent. The stone causing this threat is called a peep.
Sometimes it's hard to tell if a move is forcing, i.e. if the peep is correct or wrong. Playing correct peeps is one of the most sophisticated aspects of Hex strategy.
Another term for peep is nozoki (borrowed from Go).
Example
In this position, Red can play 2 correct peeps — c4 or c3. If Red plays c3, Blue must defend with c4. If Red plays c4, Blue must defend with c3. Otherwise, Blue loses the game.
Notice that moves at a4, a5, f2 and f3 are wrong peeps. Against a4 or a5, Blue has a choice of playing b5, which potentially improves Blue's position. Similarly, against f2 or f3, Blue should play d2.
Crucial peep
Playing peeps can be very useful. In certain situations, playing a peep can make the difference between winning and losing. Consider the following example:
The only winning move for Red is the peep at d6. If Red misses it, the game proceeds as follows and Blue wins:
If Red starts by playing d6 instead, Red wins.
