Glasses are a device that usually hold corrective lenses for those needing vision correction. One variant of the glasses is the monocle (a single corrective lens) that is worn over one eye. A form of glasses can be used to block out bright lights are called sunglasses and non-corrective glasses can be used for disguises or humor.
No One Lives Forever
While Santa was walking Cate Archer through Basic Field Tactics, he demonstrated a "cut scene" where two UNITY employees were wearing different glasses. One was a funny set of glasses and the other was a pair of sunglasses.
The Spy-Glasses was a special device developed by Santa for use by agents in the field. It contained several gadgets built into a stylish pair of sunglasses.
No One Lives Forever 2
At UNITY Headquarters, Dr. Otto Schenker was looking for his glasses (which he often misplaced) after Archer had brought him the technical data on the Super Soldiers that she had retrieved from the H.A.R.M. Antarctica Base. Archer went to the cafeteria to retrieve the lost frames, but encountered dangerous Mimes along the way.
People known to wear Glasses
Glasses on Wikipedia