We can be forgiven for confusing discrete and discreet., but they're two words that should always make us pause and think if we're copyediting a document. Both derive from a Latin word, discretus, which means separate. The discrete spelling keeps that sense; the discreet spelling means quiet and cautious.
I keep them straight in my mind by remembering that the island of Crete is a discrete part of Greece.
An editor and writer in the Facebook group Editors' Association of Earth asked for a mnemonic device to keep the two words straight, and there were several memorable suggestions. One popular way of remembering it is that the t keeps the two e’s separate and discrete, and the two e’s hide together discreetly behind the t. Or, as another editor suggested, a parakeet can be indiscreet. Or something discrete is complete in itself.
The words have an interesting history, with the discreet sense being the older of the two in English. The prudent discreet comes from French, discret, and is found in writing from the 14th century on, according to the Oxford English Dictionary. The spelling was usually discrete, but the discreet spelling was also popular.
When the original sense of the Latin word entered English in the 16th century as discrete, writers started to favor the discreet spelling for the quiet sense as a way to keep the two spellings discrete.
Discretus is a form of discernere, with the root cernere, which also meant to decide. That root also gives us discern, which presents its own spelling challenge for those unfamiliar with the word. Discern’s original sense was to separate things, and by the late 15th century it was used in the sense of recognizing things as distinct.
Other cousins that trace their roots back to the Latin word for separate include certain, concern, and excrete.