The colon (:) is one of the trickier punctuation marks. You can think of the colon in a sentence as similar to an equal sign: what follows the colon defines, explains, or expands on what came before.
One flower stands above all others in poetry, prose, and legend: the rose.
Some seed-starting mixes contain coir pith: a spongy material made from the husk of coconut, which improves air porosity and drainage.
A colon can also introduce a list.
There are five taxonomic kingdoms: Protista, Fungi, Plantae, Animalia, and Monera.
But be careful—a colon is used before a list only when the introductory clause is a complete sentence on its own. A common error is using a colon in sentences such as the first one below.
✗ The five taxonomic kingdoms are: Protista, Fungi, Plantae, Animalia, and Monera.
✓ The five taxonomic kingdoms are Protista, Fungi, Plantae, Animalia, and Monera.
Other uses of the colon include separating a title and subtitle and introducing block-set quotations.