The office where seats for a play, concert, or other form of entertainment may be purchased, as in Tickets are available at the box office. It is so called because originally (17th century) it was the place for hiring a box, a special compartment of theater seats set aside for ladies.
[ Second half of 1700s
The financial receipts from a performance; also, a show's relative success in attracting a paying audience. For example, You may not consider it great art, but this play is good box office.
[ c. 1900