Answer:
The word that bests fit the definition "spread from Arabia into Africa beginning in the A.D 600s" is ISLAM
Step-by-step explanation:
Islam is the religion which teaches that there is only one God, and the Prophet Muhammad (Peace Been Upon Him) is the last messenger of God. Islam is the world's second largest religion, with a total of more than 24% of world population as its followers.
Islam, basically started to spread from Arabia. After covering the Middle East, Islam was spread to the North Africa in 600s, when the Umayyad Caliphate conquered the area. After the conquest of North Africa, Islam spread through West Africa mainly due to peaceful means, from where it reached the rest of the Africa.