Madd (from the Arabic, madda) is a diacritic (symbol) placed over an initial (and very infrequently medial) alef, when it is vocalized (i.e., followed by the long vowel [ā])—that is, the sequence [aā].
A medial or terminal alef is by default a long [ā], as in باد bād ‘wind,’ and, therefore, it does not take a madd.
In the Arabic loanwords the madd may occur medially as well; as in, قرآن qorān (Arab. Al-qur’ān); مآب ma’āb ‘having the manner of’, normally used in compound adjectives, such as فرنگی مآب farangi ma’āb ‘having the manner of (or imitating) Europeans/Westerners.’
Sometimes in Persian madd is refered to as kolāh ‘head-gear.’