: | ː

The colon (:) is a punctuation mark used in many ways:

• to introduce lists (like this one)
• before a definition
• before a description
• to make explicit or expand on something, as exemplified on the following line:
• in Finnish and Swedish: to indicate possession (playing the role similar to that of the apostrophe in English)
• before an explanation or an example, as shown on the following line
• to form contractions, as in Swedish: Thomas Gustafsson can be abbreviated as Thomas G:son
• before direct speech or quotations. For example, Wikipedia says:

  In British English, it was once common for a colon to be followed by a hyphen or dash to indicate a restful pause, in a typographical construction known as the “dog’s bollocks”, though this usage is now discouraged.

• to introduce spoken words in written dialogues
• to separate a title and subtitle of a work when written the same line
• in American English: in formal letter writing
• to separate hours:minutes:seconds when written in numbers
• to separate chapter:verse in Biblical citations
• in mathematics and elsewhere to indicate a ratio or a scale
• in a variety of other ways in mathematics
• in certain O:B:Sc:U:Re ways in chemical nomenclature
• in many ::wonderful ways:: in C:\omputing

A triangular colon (ː) is used in International Phonetic Alphabet to mark /mɑːk/ the preceding sound as “long”.

More photos of ink and water drops @ Shutterstock.