Simple Definition:

A colon is a punctuation mark (a symbol used in writing) that is used to introduce a list, explanation, or a quotation within a sentence. It looks like two dots stacked vertically (:).

Examples:

• My favorite colors are: red, blue, and yellow. (The colon is used to introduce the list of colors.)
• She told me her secret: she is moving to a new city. (The colon is used to introduce an explanation or additional information.)

Very Simple Definition:

A colon is a symbol in writing that looks like two dots on top of each other (:). It is used to show a list or to give more information about something.

Examples:

• My favorite colors are: red, blue, and yellow.
• She told me her secret: she is moving to a new city.