The Pantheon is Greek for "Temple of All Gods," a term used by both Greeks and Romans in ancient times. Most commonly, the term "Pantheon" refers to a temple in Rome by the same name. Built in the early Roman Republic, the Temple was rebuilt by Marcus Vipsanius Agrippa, under the reign of Augustus Caesar, and features a domed ceiling with a single oculus (hole) in the very top of the dome. Around the length of the circular chamber stood statutes of the individual polytheistic gods (Jupiter (god), Venus (god), etc).
Since the coming of Christianity to Rome, the Temple has been converted to a church, and where once stood the Roman gods, now stand figures of the Virgin Mary. As a result of its repurposing, the Temple is very well preserved, and while much of Rome stands in ruin, the Pantheon gives the interested visitor a window into the way Rome may have looked, two-thousand years ago.
Alternately, the term refers to the collection of "all gods" in a polytheistic religion.