Sentence:  As Islam spread eastward throughout the eighth century AD as far as the Indus River, the Indian style of numeration began to diffuse westward and supplant the Arabic <WSD>abjad</WSD>, which itself was still a novelty in western regions such as North Africa. 
Meaning of the word marked with <WSD> tags:  The system of abjad numerals; a numeral system in which the letters of the Arabic abjad are interpreted as numerals, typically used to enumerate lists and nested lists, as well as in numerology.