A small slanting line َ above a letter in Arabic, the sign of the vowel A. To indicate a long A it is followed by the consonant alif. Note that the "th" in the name is two separate consonants, t-h. with a pharyngeal h. In the indefinite accusative ending pronounced "an" (called nunation) the N is not written, but the fathah is doubled ً, and an alif is added to masculine nouns, although the vowel is short.
In Unicode: plain fathah is &1614; and doubled fathah is ً. Here are examples with the consonant د d: