symmetry element
Hold a water molecule up to a mirror placed edge-on through the oxygen, and the reflection looks exactly like the original — you could not tell the two apart. That mirror is a symmetry element: a geometric feature (a point, a line, or a plane) about which you can move the molecule and have it land back on itself. Snowflakes, starfish, and a perfectly drawn letter H all have such features; molecules have them too, and inorganic chemists make a whole language out of them.
There are five kinds of symmetry element used to describe molecules: the identity (E, doing nothing at all), a proper rotation axis (Cn, a line you spin the molecule around), a mirror plane (sigma, a flat surface you reflect across), a center of inversion (i, a single point through which every atom is pushed to the opposite side), and an improper rotation axis (Sn, a rotation followed by a reflection). Each element is a fixed piece of geometry that stays put; it is the scaffold on which the actual moves are built.
Keep the element and the move it allows clearly apart, because the two are easy to confuse and the difference matters in everything that follows. A symmetry element is the geometric object — the axis, the plane, the point. The action you perform with it (the spin, the reflection) is the symmetry operation. One element can give rise to several operations: a single threefold axis lets you rotate by 120 degrees, by 240 degrees, and (trivially) by 360 degrees, so one line generates more than one move.
Ammonia (NH3) has a threefold rotation axis (C3) running through the nitrogen, three mirror planes (each containing the N and one H), and the identity (E). Those geometric features are its symmetry elements.
Ammonia's symmetry elements: one C3 axis, three mirror planes, and E.
Element versus operation is the single most common beginner mix-up: the element is the unmoving geometric object, the operation is the act of moving the molecule with it. They are counted differently and used differently.