A small circle ْ written above a letter in Arabic to indicate that there is no following vowel. Words rarely end with consonants in Arabic, and when they do they often take on a linking vowel when another word comes after them, so the main use of sukûn is in consonant clusters.

In Unicode: sukûn is &1618;. Here is the word sukûn itself as an example:

سُكُونْ

A similar device in Sanskrit is the mark virama.