A semicolon is a punctuation mark. It is used to join two sentences slightly more closely than they would be joined if separated by a full stop. The semicolon is also used as a sort of "senior comma" to make meaning clear in a sentence where commas are already being used for other purposes.
Examples:
I am alone; my wife had to leave.
I traveled to London, England; Tijuana, Mexico; and Reykjavik, Iceland.
Lisa scored 2,845,770 points; Marcia, 2,312,860; and Jeff, 1,726,640.
... VII., who did the fourth book.
Translations appeared in Italian (Venice, 1488,
Milan 1489), Spanish (Seville, 1536), Arabic (Rome,
1663), Armenian (Rome, 1674), ...