When someone calls you a 'unicorn'?

When you guys call someone a unicorn, or say "you're a unicorn to me" to someone you're dating/talking to what does it mean?

This guy I'm dating now calls me it as a nickname and he says it's because of my looks but I looked up "unicorn" and it seems the dating definition is a bisexual female who has sex with a straight couple? What do you guys mean if you refer to someone as that?
When someone calls you a 'unicorn'?
Post Opinion