deficient

adj

1. Lacking something essential; often construed with in.

   “They were deficient in social skills.”

2. Insufficient or inadequate in amount.

   “Apothecia lecanoroid; multilocular; each cell containing a single theke; paraphyses deficient.”

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. A person who is deficient.

   “This was justified by the rationale that mental deficients, by definition, are not able to succeed in society.”