The definite article of Arabic is الـ (Alif-Laam), known as لام التارف (Laam Al-tarif).
The first letter of the laam al-tarif is a Hamzat-Wasl (همزة وصل), an alif with a Saad (ص) written above it (ٱ). We do not write the ص on the alif in writing (we simply know it is there, as no words in Arabic begin with Alif (only Alif with Hamza (أ)). A Hamzat-Wasl takes the sound of a vowel immediately before it, if there is one. If not, it makes the same sounds a fatha at the beginning of the word (أَ).
An example of the hamzat-wasl taking the sound of a preceding vowel is الأُممُ الـمُتَّحِدة) (Al-Umamu ul-Muttahida), "The United Nations."