From BiblePortal Wikipedia

Charles Buck Theological Dictionary [1]

The place in which the ceremony of baptism is performed. In the ancient church, it is said, it was generally a building separate, and distinct from the church. It consisted of an ante-room, where the adult persons to be baptized made their confession of faith; and an inner room, where the ceremony of baptism was performed. Thus it continued to the sixth century, when the baptisteries began to be taken into the church.

Webster's Dictionary [2]

(1): (n.) In early times, a separate building, usually polygonal, used for baptismal services. Small churches were often changed into baptisteries when larger churches were built near.

(2): (n.) A part of a church containing a font and used for baptismal services.

The Nuttall Encyclopedia [3]

A circular building, sometimes detached from a church, in which the rite of baptism is administered; the most remarkable, that of Pisa.