Friends. I am confused. If semi, demi and hemi all have the meaning of "half" then how to know which should be used with this or that word? There is hemisphere and semicircle and demicycle. Will it be a mistake to say semicycle, hemicircle or demisphere? Are they interchangable? I guess there should be a strict difference between the three. And can they be used with adverbs and verbs?
Google found the following: "prefixes-semi-hemi-and-demi":
Orion Montoya, "A trifle bombastic." Lexicographer/Engineer Updated Mar 12, 2011
Hemi- is Greek; Semi- is Latin; Demi- is French. They all mean some kind of "half."
Normally when you form a new word, you're supposed to try to make all of its parts come from the same language: some pedants complain about "television" combining Greek "tele" and Latin "vision." This is not a hard and fast rule but it's the easiest* way to decide which to use, and the majority of the time it explains why one is used over another.
- "Easiest" provided you know the origin of the other parts of the word.
Many demi- words are from the time when English was getting new lexical inventory from the Normans or when French was the lingua franca. So words in heraldry, costume/fashion, arms & armor, and arts in general are likely to use demi-. I feel that forming a new or nonce word today with "demi-" is often meant to exploit this French association in one way or another (to connote fanciness or pretentiousness).
Semi- is often used on technical terms in any subject area, including music and arts and also math. Since much of technical/scientific neologism is Latinate by default, this usually works out. I intuitively feel that "semi-" is the most neutral of the three prefixes in English.
Since hemi- usually comes with Greek roots, it usually seems the most foreign, technical and scientific. Many of its applications are medical, biological, chemical or mathematic. Using hemi- for a casual word formation would likely be a bit humorous.
French itself has a prefix "mi-", which is short for "demi-" but could of course be short for any of them. You can have foie gras mi-cuit, or cheveux mi-longs. Using mi- in English would just be confusing unless you hang out with just the right kind of eurotrash.
OS X's /usr/share/dict/words list contains 951 semi- words, 215 hemi- and 172 demi-. So semi- is by far the most common.