The state of having already been reserved; can be used as a noun or modifier (e.g., 'a reservation already made').
その席はすでに予約済みです。別の席をご案内しますか？
That seat is already reserved. Would you like me to show you another seat?
予約済みのチケットは当日キャンセル料がかかる場合がある。
Reserved tickets may incur cancellation fees on the day of the event.
この時間帯は予約済みのため、他の時間をお選びください。
This time slot is already booked, so please choose another time.