This is the 7465th most frequent Chinese word.
補選
A "by-election" held to fill a vacancy in an elected position.
In this sentence, '補選' refers to a by-election conducted to elect someone to fill a vacant position.
明天我們要舉行學生會長的補選。
Tomorrow we'll hold a by-election for the student council president.
Here, '補選' is used in the context of electing a new official after a resignation.
如果市長辭職，將必須舉行補選。
If the mayor resigns, a by-election will have to be held.
In this example, '補選' describes the action of choosing a replacement representative.