there are lots of definitions of love. sternberg came up with 7 different types, depending on 3 things: committment, intimacy & passion. (find the power set of this set & throw away the empty set
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here's a description on good ol wikipedia:
http://en.wikipedia.org/wiki/Triangular_theory_of_love
1. Liking includes only one of the love components - intimacy. In this case, liking is not used in a trivial sense. Sternberg says that this intimate liking characterizes true friendships, in which a person feels a bondedness, a warmth, and a closeness with another but not intense passion or long-term commitment.
2. Infatuated love consists solely of passion and is often what is felt as "love at first sight." But without the intimacy and the commitment components of love, infatuated love may disappear suddenly.
3. Empty love consists of the commitment component without intimacy or passion. Sometimes, a stronger love deteriorates into empty love, in which the commitment remains, but the intimacy and passion have died. In cultures in which arranged marriages are common, relationships often begin as empty love.
4. Romantic love is a combination of intimacy and passion. Romantic lovers are bonded emotionally (as in liking) and physically through passionate arousal.
5. Companionate love consists of intimacy and commitment. This type of love is often found in marriages in which the passion has gone out of the relationship, but a deep affection and commitment remain.
6. Fatuous love has the passion and the commitment components but not the intimacy component. This type of love can be exemplified by a whirlwind courtship and marriage in which a commitment is motivated largely by passion, without the stabilizing influence of intimacy.
7. Consummate love is the only type of love that includes all three components--intimacy, passion and commitment. Consummate love is the most complete form of love, and it represents the ideal love relationship for which many people strive but which apparently few achieve. Sternberg cautions that maintaining a consummate love may be even harder than achieving it. He stresses the importance of translating the components of love into action. "Without expression," he warns, "even the greatest of loves can die"
there's more info in mark knapp's interpersonal comminication in human relationships, and more still in sharon brehm's intimate relationships.
another definition that i saw in one of those books is that someone is in love when things that happen to the other person become as important as things that happen to themselves.
*snicker*