A colon is a punctuation mark, with one dot above another, like this: ":".

Colons are commonly used to introduce lists, or to connect a broad idea with a specific example: two related sentences can be separated by colons instead of periods. A colon can only be used if the clause preceding the colon is independent.

Examples

The United Kingdom comprises four countries: England, Wales, Scotland and Northern Ireland.

Speech is silver: silence is golden.

She had not eaten since breakfast: she'd worked through her lunch break.

Also use the colon...

After the salutation of a business letter (in US practice, a comma being preferred in many other cultures).

In the heading of a business memo.

Between the hour and the minutes when telling time (though a full stop is sometimes used instead).

Between chapter and verse in the Bible, between volume and number in publications, and between title and subtitle.

Mathematics

The colon is also used in mathematics to indicate ratio, and is also the standard sign for division in most non-English-speaking countries. In mathematical logic the colon is often used to represent "such that" in a relational phrase from predicate calculus. Unicode provides ratio U+2236 (∶, ∶) for such mathematical usage if the distinction is required.

Time

The colon separates the hour from the minute (though the full stop is often used instead).

Linguistics

A special triangular colon symbol is used in IPA to indicate a preceding long vowel. It is available in Unicode as Modifier letter triangular colon Unicode U+02D0 (ː, ː). A regular colon is often used as a fallback when this character is not available.

Computer representation

In computer science, the colon character corresponds to the decimal value 58 (hexadecimal value 3A) in Unicode and ASCII character encodings.

Other meanings

See Colon, the disambiguation page.