A conventional defence to an opening three-bid. Using this convention, a bid of the lower unbid minor (3♦ over 3♣ or 4♣ over anything else) is a take- out request.
The advantage of this defence is that you can use all other suit overcalls, 3NT and a double in their natural sense. The disadvantages are that you can rarely stop below the four level and that you cannot pick up a penalty when one member of the partnership has good values and the other holds good trumps. Most pairs now prefer to use double for take-out.