A decimal is a number expressed in the base-10 numeral system, using a decimal point to separate whole and fractional parts.
Decimals represent numbers using place values of powers of 10: units, tenths, hundredths, thousandths, etc. Repeating decimals have a pattern that repeats infinitely (like 1/3 = 0.333...). Terminating decimals end after a finite number of digits (like 1/4 = 0.25). Decimals are used in currency, measurements, and scientific notation.
The decimal system originated in India around the 5th century CE, with place-value notation using nine digits and zero. Persian mathematician al-Khwarizmi helped spread the system to the Islamic world in the 9th century. The decimal point was introduced by Italian mathematician Giovanni Bianchini in the 1440s and popularized by John Napier and Simon Stevin in the 16th-17th centuries.