Classical Arabic

Hamzah ء — glottal stop

Hamzah can appear on its own, or be above the letter or below the letter.

Hamzat al-Waṣl — connecting hamzah ()

Hamzat al-Waṣl is a small ṣād (ص) on top of alif (ا). It means the alif is not pronounced.

elidable alif

It only occurs in the beginning of words (can occur after prepositions and the definite article). It is commonly found in imperative verbs, the perfective aspect of verb stems VII to X and their verbal nouns (maṣdar). The alif of the definite article is considered a waṣlah.

http://www.abouttajweed.com/hamzah_al-wasl_6.htm

http://en.wikipedia.org/wiki/Arabic_diacritics#Alif_waslah