The clear aperture is the pupil of the system that preserves all specifications defined by the manufacturer. It is always smaller than the diameter of the optic.
Coatings are used to alter the reflective properties of a surface. They can be used to increase transmission (AR Coatings), increase reflection (HR Coatings) or selectively transmit or reflect certain wavelengths.
The CW damage threshold indicates the maximum power density that an optic can withstand without being damaged by a CW laser. Power Density (W/cm2) = P (W) / A (cm2).
Thickness indicates the distance between two surfaces. Larger thickness allows for greater heat dissipation and higher resistance to stress in machining, blocking, and handling, however it increases weight and dispersion.