The first card led by defenders in the play stage. Unlike other leads, the dummy is not uncovered yet at the time of lead, so the opening lead is made "blindly", and often has a crucial impact to the outcome of the deal.
The opening lead is the first card played in the playing phase of a contract bridge deal. The defender sitting to the left (LHO) of the declarer is the one who makes the opening lead. Since it is the only card played while dummy's cards are still concealed, it can be critical for the outcome of the deal.