This word rarely sees the light of day. Its only recent outing I can find was in the 2001 National Spelling Bee in the US, but then the easy words, like the poor spellers, get weeded out early in that contest.
You will find it only in the larger dictionaries. It is defined in two slightly different ways. One is that it is the science of defining technical terms, but it is also used less rigorously as an alternative for terminology. If you go by the original Greek (which is always a dangerous move), you have to accept the first definition, as it comes from horismos, a definition (literally, the marking of a boundary, related to our horizon) plus the –ology ending for a field of study.
The first persons to use it were the pre-eminent experts on insects in the early nineteenth century, William Kirby and William Spence. They didn’t like the way that in terminology a Latin stem had been joined to a Greek suffix, so in their textbook in 1816 they created orismology as an alternative.
They were better entomologists than etymologists, since the word really ought to have an initial h. The linguistic purity of orismology was not enough to ensure its widespread adoption, and it now languishes as a hard word for expert spellers and a topic of pieces like this one.
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