The 18th day of a month (used for dates).
１８日は出張で不在です。
I will be away on business on the 18th.
次のイベントは６月１８日に行われます。
The next event will be held on June 18th.
１８日に結果が発表されます。
The results will be announced on the 18th.
A duration of eighteen days (a period lasting 18 days).
研修は１８日間にわたって行われる。
The training will take place over eighteen days.
１８日間の旅行から戻ったばかりだ。
I just returned from an eighteen-day trip.
工事は約１８日で完了する予定です。
The construction is expected to finish in about eighteen days.