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.