Identifying another person as physically attractive, then stating if the chance presented itself, you would fornicate with them. There is no prerequisite to being an ILF except that you must NOT be a mother, grandmother, or disabled (MILF, GILF, DILF). ILF is limited to those that don't fit under other ILF categories.
(As an attractive female walks by, you point and exclaim) "ILF!"
"Ey, lets go to that party, I heard there'll be a grip of ILFs there!"