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Peter I wouldn't use either term - they're both open to interpretation - that you're even asking confirms this. 'Optical centre' is a bit meaningless. 'Geometric centre' or just plain 'centre' could just as easily mean the centre of the mirror surface, right where many place a collimating dot.
If you mean the focus, that is the correct word to use. If you mean the "centre of curvature", ie located at 2F in front of the surface, that is also unambiguous.
Note that 'focus' is strictly only valid in the context of a parabolic mirror, and it is also valid in the context of a complete lens. In the case of a mirror that is some other shape (spherical, ellipsoidal, hyperboloidal) 'focus' should be avoided; radius of curvature is more often used by the optical designer or mirror maker. Similarly for individual surfaces on a lens, the optical designer calculates (and specifies) the radii and this is also what the optician will measure when making one.
Hope this helps !
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