In ancient Rome, thermae (from Greek θερμός thermos, "hot") and balneae (from Greek βαλανεῖον balaneion) were facilities for bathing. Thermae usually refers to the large imperial bath complexes, while balneae were smaller-scale facilities, public or private, that existed in great numbers throughout Rome.

Most Roman cities had at least one – if not many – such buildings, which were centers not only for bathing, but socializing and reading as well. Bathhouses were also provided for wealthy private villas, town houses, and forts. They were supplied with water from an adjacent river or stream, or within cities by aqueduct. The water would be heated by fire then channelled into the caldarium (hot bathing room). The design of baths is discussed by Vitruvius in De architectura (V.10).

Thermae, balneae, balineae, balneum and balineum may all be translated as 'bath' or 'baths', though Latin sources distinguish among these terms.

Balneum or balineum, derived from the Greek βαλανεῖον signifies, in its primary sense, a bath or bathing-vessel, such as most persons of any consequence among the Romans possessed in their own houses, and hence the chamber which contained the bath, which is also the proper translation of the word balnearium. The diminutive balneolum is adopted by Seneca to designate the bathroom of Scipio in the villa at Liternum, and is expressly used to characterize the modesty of republican manners as compared with the luxury of his own times. But when the baths of private individuals became more sumptuous and comprised many rooms, instead of the one small chamber described by Seneca, the plural balnea or balinea was adopted, which still, in correct language, had reference only to the baths of private persons. Thus, Cicero terms the baths at the villa of his brother Quintus balnearia.

Balneae and balineae, which according to Varro have no singular number, were the public baths, but this accuracy of diction is neglected by many of the subsequent writers, and particularly by the poets, amongst whom balnea is not uncommonly used in the plural number to signify the public baths, since the word balneae could not be introduced in a hexameter verse. Pliny also, in the same sentence, makes use of the neuter plural balnea for public, and of balneum for a private bath.

Thermae (Greek: Θέρμαι, Thermai, 'hot springs, hot baths', from the Greek adjective thermos, 'hot') meant properly warm springs, or baths of warm water; but came to be applied to those magnificent edifices which grew up under the empire, in place of the simple balneae of the republic, and which comprised within their range of buildings all the appurtenances belonging to the Greek gymnasia, as well as a regular establishment appropriated for bathing. Writers, however, use these terms without distinction. Thus the baths erected by Claudius Etruscus, the freedman of the Emperor Claudius, are styled by Statius balnea, and by Martial Etrusci thermulae. In an epigram by Martial—subice balneum thermis—the terms are not applied to the whole building, but to two different chambers in the same edifice.

A public bath was built around three principal rooms: the tepidarium (warm room), the caldarium (hot room), and the frigidarium (cold room). Some thermae also featured steam baths: the sudatorium, a moist steam bath, and the laconicum, a dry hot room.^{[citation needed][dubious – discuss]}

By way of illustration, this article will describe the layout of Pompeii's Old Baths, otherwise known as the Forum Baths, which are among the best-preserved Roman baths. These baths were connected to the forum at Pompeii, hence the name. The references are to the floor plan pictured to the right.