The story
From midterm + break. From being a break in schooling during the middle of a term (semester).
Etymology from Wiktionary, CC BY-SA 4.0.
midterm break
Today it means: A break (frequently, week-long) in classes taking place somewhere between the end of the first third and start of the last third of a semester (term), this may coincide with a mid-semester examination period (midterms). Full definition →