What is "deficient number"

Deficient \De*fi"cient\, a. [L. deficiens, -entis, p. pr. of deficere to be wanting. See Defect.] Wanting, to make up completeness; wanting, as regards a requirement; not sufficient; inadequate; defective; imperfect; incomplete; lacking; as, deficient parts; deficient estate; deficient strength; deficient in judgment.

The style was indeed deficient in ease and variety.
--Macaulay.

Deficient number. (Arith.) See under Abundant. -- De*fi"cient-ly, adv.

n. (context number theory English) A number that greater than the sum of all of its divisors except itself.

In number theory, a deficient or deficient number is a number n for which the sum of divisors σ(n)<2n, or, equivalently, the sum of proper divisors (or aliquot sum) s(n)<n. The value 2n − σ(n) (or n − s(n)) is called the number's deficiency.