An obelisk is a thin, four-sided, tapering monument which ends in a pyramidal top. Ancient obelisks were made of a single piece of stone (a monolith).
Obelisks were erected by the ancient Egyptiansians, placing them in pairs at the entrance of temples, but apparently only 27 large-scale standing obelisks have survived (see link below). The Romans were infatuated with obelisks, to the extent that there are now more than twice as many obelisks standing in Rome as remain in Egypt.
Not all the Egyptian obelisks re-erected in the Roman Empire were set up at Rome. Herod the Great imitated his Roman patrons and set up a red granite Egyptian obelisk in the hippodrome (racetrack) of his grand new city Caesarea in northern Palestine. It was discovered by archaeologists and has been re-erected at its former site.
In Byzantium, the Eastern Emperor Theodosius shipped an obelisk in 390 CE and had it set up in his hippodrome, on a specially-built base, where it has weathered Crusaders and Seljuks and stands in the Hippodrome square in modern Istanbul.
Rome is the obelisk capital of the world. The most prominent must be the obelisk at St. Peter's Square in Rome, the re-erection of which, by Fontana, was a famous feat of 17th century engineering, that Fontana detailed in a book illustrated with engravings. Another obelisk stands in front of the church of Trinità dei Monti, at the head of the Spanish Steps. There is a further famous obelisk in Rome, sculpted as carried on the back of an elephant. Rome lost one of its obelisks, which had decorated the temple of Isis, where it was uncovered in the 16th century. The Medici claimed it for the Villa Medici, but in 1790 they managed to move it to the Boboli Gardens attached to the Pitti Palace in Florence, and left a replica in its stead.
Several more of the original Egyptian obelisks have been shipped and re-erected all over the world. The best-known examples outside Rome are the pair of so-called Cleopatra's Needles in London and New York and the obelisk at the Place de la Concorde in Paris.
The Ethiopian Obelisk of Axum was looted and taken to Rome in 1935; in 2003 the Italian government agreed to return it.
More obelisks are listed at Monolith.
The Washington Monument is a modern obelisk.