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.
A number is deficient if sigma(n) 2n, or, equivalently, if (n) 2. Another way of saying this is that sum of the proper divisors of n is less than n. Contrast abundant, perfect. Every divisor of a deficient number is deficient.