deficient

Short definition from Wiktionary

Adjective

1. 1
   Lacking something essential; often construed with in.
2. 2
   Insufficient or inadequate in amount.
3. 3
   Of a number n, Having the sum of divisors σ(n)<2n, or, equivalently, the sum of proper divisors (or aliquot sum) s(n)<n.

Noun

1. 1
   A person who is deficient.