life in sente
In terms of life and death, the best outcome for a group of stones is life in sente.
A move is sente if the opponent has to answer it, so the player who plays it will maintain sente after the exchange. Life in sente requires the move which both requires a response and causes a group of stones to live.
(From Graded Go Problems For Beginners, Vol. 4, #23)
To protect against black capturing the corner by moving at a, white must respond to . Black then plays elsewhere; black can capture marked white stones whenever he wants.
Black captures white's stones, but white doesn't need to respond to this move, and can play elsewhere.