When should the word mid be hyphenated?

Answer 1

Generally, hyphenate the "mid" (within, in the middle of) prefix if it modifies an original word as opposed to a distinct term, and for items that do not have separate "ends" or "middles". e.g.

midday, midair, midway, but mid-sentence, mid-ocean, mid-Atlantic, mid-stride. Also, if the resultant word appears indistinct, such as "midriver" (readers see the word 'driver'), hyphenation may clarify the intent.

Answer 2

The word "mid" should be hyphenated when it is being used as a prefix followed by a capitalized word (e.g., mid-September, mid-July). Otherwise, "mid" does not require a hyphen when used on its own or with lowercase words (e.g., midmorning, midterm).