A stoa is a type of building in ancient Greek architecture that was commonly used for public purposes. It is a covered walkway or portico that was open at the entrance with columns, usually of the Doric or Ionic order, lining the side of the building. Stoas were usually built near marketplaces or agoras of large cities and were used as a framing device. They were also used as galleries for art and public monuments, were used for religious purposes, and delineated public space. Some stoas were designed to create safe, protective atmospheres which combined useful inside and outside space. Stoas were often two stories high and incorporated inner colonnades usually in the Ionic style, where shops or sometimes offices were located. Rooms might back onto the colonnade, and a second story was sometimes added.

The most famous stoa is the Stoa Poikile, also known as the Painted Porch, from which the philosophy Stoicism takes its name. Other famous stoas include the Stoa of Attalos, Stoa Basileios (Royal Stoa), Stoa of Zeus at Athens, Stoa Amphiaraion, Stoa of the Athenians, and Royal Stoa of Herods Temple.

In summary, a stoa is a type of building in ancient Greek architecture that was used for public purposes, such as marketplaces, religious gatherings, and public promenades. It is a covered walkway or portico that was open at the entrance with columns lining the side of the building. Stoas were usually two stories high and incorporated inner colonnades usually in the Ionic style, where shops or sometimes offices were located.