One third is a fractional unit, which is a mathematical term. The unit "1" is divided into several parts, and one of them is called fractional unit, that is, the fraction whose numerator is 1 and denominator is a positive integer, which is also called unit fraction, and it is also called 1/n, which is also called "monomolecular fraction".

General fractions are calculated by converting fractions with different denominators into numbers with the same decimal unit. For example, the decimal unit of one eighth is one eighth, and so on. Fractions are equal in size, but the fractional units are not necessarily equal. If two eighths equals a quarter, a quarter has a large decimal unit. The maximum number unit is half, and there is no minimum number unit. The molecular size of a fraction is constant, and the larger the denominator, the smaller the fractional unit; In the case of the same molecular size, the smaller the denominator of a number, the larger its fractional unit.