All these are equivalent,
1/ Aperture of a spherical mirror is the diameter of the reflecting surface, the straight line distance between two farthest point on the periphery of the mirror.
2/ In general aperture refers to a tiny hole, you'll encounter this in diffraction, and interference phenomena.
3/ What's the measure of the area exposed to the incident light, it's the area of the mirror, and it's measure is the diameter from which we can obtain the area.
You have rated this answer /10