Guilty as charged, I've always called them monocles...

... thereby branding myself as gauche.
So be it.
Gerald Warner tells us that the single eyeglass, ("vulgarly known as [a] monocle,") is being sold in what seems to me to be the British version of Lenscrafters or For Eyes.

Apparently it is a youthful fashion, in which case it will not last. It may cause some perplexity among the relentless dressers-down in Cameron circles and raise fears of a wave of reactionary opinion.

Yet, only six years ago, there was outrage among genuine eyeglass wearers when Dollond & Aitchison announced it was no longer going to stock “monocles” except in its main London store. The lore of this essential optical aid is fascinating and longstanding. ...

When the Irish Free State in 1932 increased the tax on monocles by 50 per cent, this was seen as an act of petty revenge against the fallen Ascendancy. ... “eyeglass” is U, “monocle” Non-U. The eyeglass permeates high literature, from Proust up to P G Wodehouse. Its significance varies with nationality.

An Englishman traditionally favours a gold-rimmed eyeglass with a gallery to hold it in place, attached to a black cord (my own practice). The degenerate French seducer will most likely sport his on a broad ribbon.

Go read the rest, as I follow his link in order to read his self-proclaimed "definitive account of the history and significance of the eyeglass."

Wonderful line! It is a Cavalier accessory in an increasingly Roundhead world.

I do love a monocle... Himself has two. (And a pince-nez.) He looks like Richard Tauber in one.