# Javascript, 21 characters

The standard rule is that `y` is a leap year if 4 divides `y` and if either 100 doesn't divide `y` or 400 does divide `y`. In code,

    y%4 == 0 && (y%100 != 0 || y%400 == 0)

There's no need for that 100 and 400. Instead it suffices to check whether 16 or 4 divides y, with 16 chosen if 25 divides y, 4 otherwise. Golfed, this becomes

    !(y%(y%25?4:16))

A javascript function that implements this is 21 characters long:

    l=y=>!(y%(y%25?4:16))