A play designed to prevent a particular opponent from gaining the lead. This usually involves playing a suit slightly differently to normal. For example:
As South, you play in 3NT and receive a spade lead won in dummy. You wish to avoid a spade through your king- jack, which affects how you play the diamonds. You lead low from dummy and finesse the nine. This way, you will usually keep East off lead unless he holds the queen and ten of diamonds.