There are a lot of different expressions of it.

But I'd define it ultimately as this: Seeing others as yourself. Inseparably. You look at them, and you don't see them as separate, you see yourself. That's complete empathy. If you experience that, you cannot help but treat others with total love.

This is the characteristic that could be found in a fully developed spiritual master...and in God. Since God is (presumably) both infinite and omnipresent, then God must be one with all that is. No sense of separation. No sense of denial or judgement. That's love.

In our relationships we love according to the degree that we feel "at one" with the other. This is why a mother's love for a young child may be the strongest love usually seen in human relationships. The child becomes like an extension of the mother's own being. She protects it as naturally as she would her own limbs. That's love. Love is that which recognizes no separation.