Telemetry is interesting as a word not so much due to its origins, which the OED tells us is a pretty straightforward combination of a couple commonly used roots: “tele” and “metry” (measure from a distance).

It’s interesting because when you look it up in the Associated Press’s preferred dictionary, the Webster’s New World College Dictionary, the word is only listed as an alternative spelling of the noun “telemeter,” which is, foremost, an instrument for determining the distance to a remote object. It’s only in the secondary definition that you see the more common usage (if you can call any use of “telemeter” common): a device for transmitting measurements to a distance observer.

“Telemetry” is listed as an alternate noun form of that main word, not as a listed word of its own.

Interestingly as well if you search for the word “telemeter” in the Associated Press Stylebook’s online version of that New World College Dictionary, you won’t find any “telemetry” at all. It seems there’s a typo, and the adjective form, “telemetric” is repeated instead of “telemetry,” which appears in the paper dictionary.

Fun fact, a “telemeter” was also the name for the system that scrambled TV signals until money was deposited in a box next to the TV — as you might have seen in old movies featuring cheap motel rooms.