Picture a stretch of time with a clear start and a clear end, given to someone in advance: one school semester, or the years an elected government has been voted in for. English calls that stretch a term. Now put a mark exactly halfway along it. Whatever sits on that mark is midterm — the test halfway through the semester, the election halfway through a president's four years, the moment when everyone stops and asks how it is going so far.
From the word’s history
The word is formed from mid- plus term, so its parts already say halfway through a set period; term itself is used both for a school semester and for the years an official holds office, which is why one word covers school and politics.
這個字由 mid-（中間）加上 term 組成，字面就是「一段期間的中點」。term 本身既能指學期，也能指任期，所以同一個字才會同時落在學校和政治兩邊。
Wiktionary etymology supplied in this packet (kaikki:3f6f69a190036ae4): 'From mid- + term.'
登入就可以看每一支怎麼連回核心畫面。