Frederick Buechner, Wishful Thinking, A Theological ABC, (Harper SanFrancisco, A Division of Harper Collins Publishers, 1973), p. 95
It comes from the Latin vocare, to call, and means the work a man is called to by God.
There are all different kinds of voices calling you to all different kinds of work, and the problem is to find out which is the voice of God rather than of Society, say, or the Superego, or Self-Interest.
By-and-large a good rule for finding out is this.
Neither the hair shirt nor the soft berth will do. The place God calls you to is the place where your deep gladness and the world's deep hunger meet.