(Some of the definitions/assertions I've come across (and am inclined to agree with) or I've come up with myself. Obviously not comprehensive, or necessarily universal. Would be interested to hear others' opinions on the subject)
- when someone else's happiness is essential to your own.
- an act of will - a decision to behave in a certain way towards someone.
- when you look at the world through (your approximation of) their perspective as well as your own.
- something which, if you've reached the stage of seriously asking whether you're in it and you're not sure, you probably are.
- feeling like someone fills gaps in you which you never even knew were missing.
- an absolute, not measurable, not directly comparable across either time or people.
- being close enough to someone to see that the shades of their personality are as diverse and complex as your own.