The definition I give for grace in One-Way Love comes from Paul Zahl:
Grace is love that seeks you out when you have nothing to give in return. Grace is love coming at you that has nothing to do with you. Grace is being loved when you are unlovable…. The cliché definition of grace is “unconditional love.” It is a true cliché, for it is a good description of the thing. Let’s go a little further, though. Grace is a love that has nothing to do with you, the beloved. It has everything and only to do with the lover. Grace is irrational in the sense that it has nothing to do with weights and measures. It has nothing to do with my intrinsic qualities or so-called “gifts” (whatever they may be). It reflects a decision on the part of the giver, the one who loves, in relation to the receiver, the one who is loved, that negates any qualifications the receiver may personally hold…. Grace is one-way love.