Lacking something essential; often construed with in. / Insufficient or inadequate in amount.

A person who is deficient.

In a deficient manner.

The quality of being deficient.

plural of deficient

(uncountable) Inadequacy or incompleteness. / (countable) An insufficiency, especially of something essential to health.

plural of deficiency

(obsolete) Deficiency.

plural of deficience

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