"Love is that condition in which the
happiness of another is conditional to your own." Robert Heinlein from the novel "Stranger in a Strange Land" (1961)
I prefer saying "Love is that condition whereby the
happiness of another is essential to your own."
I have yet to see any better
definition, and I have been applying this one as an basic test for over 30 years. The definition works.
Basic tests:
1. If another is unhappy and you are happy, you do not love them.
2. If another is unhappy and you cannot be happy because of that, you love them.
3. If you love someone who is happy, you CAN be happy, but your
happiness is not essential,
important, or required.