The waṣla or hamzatu l-waṣli is a letter in the Arabic alphabet resembling part of the letter ṣād that is sometimes placed over the letter ʾalif at the beginning of the word. Wikipedia

A diacritic added to the Arabic letter alif ا‎ to form ٱ‎, also called "hamzatu l-waṣli" - هَمْزَةُ الْوَصْلِ‎ (hamzatu l-waṣli) or "alifu l-waṣli" - أَلِفُ الْوَصْلِ‎ (ʾalifu l-waṣli), denotes elision, seldom used in practice in fully vocalised Arabic texts, only to highlight that the alif is silent هَمْزَةُ ٱلْوَصْلِ‎ (hamzatu l-waṣli) = هَمْزَةُ الْوَصْلِ‎ (hamzatu l-waṣli). Wiktionary

An Arabic diacritic denoting elision of the alif. Wiktionary (translation)