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.
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.'
Two branches
學期的中點：考試與成績
學校把一年切成 semester 或 quarter，每一段都是一個 term。走到一半就要驗收一次，於是那個記號上放的是一場考試、一份成績單，有時候還有幾天假。這一家最貼近字面，也是台灣學生最常遇到的一家。
noun · B1
A test that students take about halfway through a semester or quarter, usually covering only the work of the first half of the course. Countable, and mostly American English.
期中考
任期的中點：政治與選舉
term 的另一個常見意思是任期。把同一條時間線換成民選政府的四年，中間那個記號一樣存在，只是站在上面的不是考試，而是選舉、民調和檢討。時間線的畫面沒變，換的只是誰被檢查。
adjective · B2
Happening in the middle of the period that an elected government or official has been voted in for — most often in midterm elections, and also in midterm results, a midterm campaign or a midterm poll.
任期中段的