How do you distinguish definitions from theorems?

I read very often and i highlight definitions in orange and theorems in purple, but sometimes I cant really distinguish a definition from a theorem, for example:

Let $A$ be a finite set and $B$ a nonempty set. $|A|≥|B|$ if and only if there exists a function that maps A onto B.

$|A|$ and $|B|$ represent the cardinality of $A$ and $B$ respectively.

I don't understand if this is a theorem (or a rule in general), or a definition, how do you distinguish definitions from theorems since both usually use the sentence "if and only if"?

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