tall, four-sided, narrow tapering monument which ends in a pyramid-like shape at the top
An obelisk is a monument made out of stone, much like a column but supporting nothing.
Obelisks were erected by Ancient Egyptians for commemoration. Obelisks were usually made of a single piece of stone.
A bird's eye view of obelisk is square and this structure has gradually tapering four sides and its ends in a pyramidal top.
Obelisks originated with the holy stone of sanctuary which served the sun god, Ra, in Heliopolis. There usually was a pair in front of a gate or sanctuary, and on each side of obelisk there were written hymns and some actions of the of king in hieroglyphics.
Obelisks can have interior spaces.
There are similar forms of obelisks in Europe and America, and some were taken from Egypt by Europeans and Americans. The Washington Monument is a large, modern, built obelisk.