perfect number

noun

: an integer (such as 6 or 28) the sum of whose integral factors including 1 but excluding itself is equal to itself

Word History

First Known Use

14th century, in the meaning defined above

Kids Definition

perfect number

noun

: a whole number that is equal to the sum of all its divisors except itself

28 is a perfect number because it is the sum of 1 + 2 + 4 + 7 + 14