The status of prefix means that mid- forms one word in combination, unless it is joined to a capital letter or a numeral, in which case a hyphen is employed: midsentence, midcentury; but mid-July, mid-1985.

Which is correct midyear or mid year?

noun. mid·​year | \ ˈmid-ˌyir \

Is midterm one word or two?

The middle of a term, therefore, is a midterm, from mid-, from a Proto-Indo-European root meaning "middle," and term, from the Old French terme, "duration."