An Etymological Dictionary of Astronomy and Astrophysics

A number that is divisible by the sum of its digits.
For example, 18 is a Harshad number because 1 + 8 = 9 and 18 is divisible by 9 (18/9 = 2).
The simplest Harshad numbers are the two-digit Harshad numbers: 10, 12, 18, 20, 21, 24, 27, 30, 36, 40, 42, 45, 48, 50, 54, 60, 63, 70, 72, 80, 81, 84, 90. They are sometimes called Niven numbers.

See also: The name Harshad was given by Indian mathematician Dattaraya Kaprekar (1905-1986) who first studied these numbers. Harshad means “joy giver” in Sanskrit, from harṣa- “joy” and da “to give,” → datum.