A small curve ُ with a loop at the right, written above a letter in Arabic as the sign of the vowel U. To indicate a long U it is followed by the consonant waw. In the indefinite nominative ending pronounced "un", the N is not written, and either the dammah is written above itself upside-down, or the smooth curve is changed to wavy ٌ. The D in dammah is emphatic, so it is pronounced d!ommah.
In Unicode: plain dammah is &1615; and doubled dammah is ٌ. Here are examples with the consonant د d: