Due to the resignation of the MP, by-elections will be held next week to fill the vacant seat.

by-elections 是由前缀 by-（表示"附属的、次要的"）和名词 elections（选举）组成的复合词。 elections 是 election 的复数形式，election 指正式的"选举"活动。 by-elections 特指"补选"，即在正式选举之外，因职位空缺而举行的额外选举。