Glenoid, from the the Greek glene, socket, eyeball, or mirror, and eidus, shape.

The etymology of glenoid, as in the glenoid cavity of the scapula, is murky at best. The earliest recorded use of glene, by Homer around the 7th century BC, was for mirror. By a few hundred years later, perhaps because of the mirror-like reflections that can be seen in the pupil, glene took on the meaning of eyeball (or, as some maintain, just the pupil). Still later, the meaning shifted to socket, perhaps because of the association of the eyeball with the orbit, or eye socket, of the skull.

But why is the socket on the scapula called the glenoid cavity? It has been suggested that the inspiration for this usage was the mirror-like glistening of the relatively flat articular cartilage on surface of the cavity, harking back to the original use of glene for mirror, which, along with its other meaning of socket, provided a term with definitions doubly appropriate to the structure. The only problem with this convenient story is a complete lack of evidence in its favor, but no better explanation has been offered.

Galen, in the second century AD, was the first to employ the word for the scapular structure, but he kept the logic for the coinage to himself.

Anatomically unrelated: The name of the protozoan euglena comes from eu- (true) and, for some reason, glena (mirror, eyeball, pupil, socket -- take your pick)