Atrium (pl. atria), one of two entry chambers of the heart. In ancient Rome, an atrium was a home's main living space, distinguished by a large open skylight that let in light and rain water (the water was collected in a central pool). Smoke from a open-hearth fireplace left through the opening, but absent a true chimney, the room had a sooty appearance. Indeed, atrium is probably derived from ater, the Latin word for black.
Given their resemblance to the Roman room, particularly the open design, early Christians used the term atria for the courtyards that served as basilica entries; thus the link to the concept of an entrance into another room or space.
The anatomical use of atria is relatively recent, apparently coined by English doctors in the 1870s. The chambers had been called the auricles and continued to be so well into the 20th century. Auricle is now properly reserved for the ear-like appendages of the atria.
A ancient Roman atrium
The entrance atrium (foreground) of Santa Sabina Basilica on Aventine Hill in Rome.