Lacking something essential; often construed with in.
Insufficient or inadequate in amount.
Of a number n, Having the sum of divisors σ(n)<2n, or, equivalently, the sum of proper divisors (or aliquot sum) s(n).
A person who is deficient.
Required as a prior condition of something else; necessary or indispensable.
Something that is required as necessary or indispensable, or as a prior condition of something else.
In education, a course or topic that must be completed before another course or topic can be started. May be colloquially referred to as a prereq.