A vestibule (also anteroom, antechamber, outer room, air-lock entry or foyer) is a small room leading into a larger space such as a lobby, entrance hall, or passage, for the purpose of waiting, withholding the larger space from view, reducing heat loss ( air trap, windbreak), providing storage space for outdoor clothing ( mudroom), etc. The term applies to structures in both modern and classical architecture since ancient times.

In antiquity, antechambers were employed as transitional spaces leading to more significant rooms, such as throne rooms in palaces or the naos in temples. In ancient Roman architecture, a vestibule (Latin: vestibulum) was a partially enclosed area between the interior of the house and the street.

In modern architecture, a vestibule is typically a small room next to the outer door and connecting it with the interior of the building.

Vestibules were common in ancient Greek temples. Due to the construction techniques available at the time, it was not possible to build large spans. Consequently, many entranceways had two rows of columns that supported the roof and created a distinct space around the entrance.

In ancient Greek houses, the prothyrum was the space just outside the door of a house, which often had an altar to Apollo or a statue, or a laurel tree.

In elaborate houses or palaces, the vestibule could be divided into three parts, the prothyron (πρόθυρον), the thyroreion (θυρωρεῖον; lit. 'porter's lodge'), and the proaulion (προαύλιον).

The vestibule in ancient Greek homes served as a barrier to the outside world, and also added security to discourage unwanted entrance into the home and unwanted glances into the home. The vestibule's alignment at right angles of private interior spaces, and the use of doors and curtains also added security and privacy from the outside. The Classical Period marked a change in the need for privacy in Greek society, which ultimately led to the design and use of vestibules in Greek homes.

In ancient Roman architecture, where the term originates, a vestibule (Latin: vestibulum) was a space that was sometimes present between the interior fauces of a building leading to the atrium and the street. Vestibules were common in ancient architecture. A Roman house was typically divided into two different sections: the first front section, or the public part, was introduced with a vestibule. These vestibules contained two rooms, which usually served as waiting rooms or a porters' lodge where visitors could get directions or information. Upon entering a Roman house or domus, one would have to pass through the vestibule before entering the fauces, which led to the atrium.