Algorism is the name for the Indo-Arabic decimal system of writing and working with numbers, in which symbols (the ten digits 0 through 9) are used to describe values using a place-value system (positional notation), where each symbol has ten times the weight of the one to its right.

This system was originally invented in India in the 6th century CE (or earlier), and was soon adopted in the Islamic world. Muslim mathematicians made many contributions (including the concept of the decimal fractions as an extension of the notation, which led to the notion of the decimal point), and the written European forms of the digits called Arabic numerals are derived from the ghubar (sand-table or dust-table) numerals used in north-west Africa and Spain.

The word algorism comes from the name al-Khwarizmi ("the one from Khwarizm"), the cognomen (nickname) of an early 9th century mathematician, possibly from what is now Khiva in western Uzbekistan. His name is also the root of the word algorithm.