the state of needing something that is absent or unavailable
"there is a serious lack of insight into the problem"
"water is the critical deficiency in desert regions"
"for want of a nail the shoe was lost"
lack of an adequate quantity or number
"the inadequacy of unemployment benefits"
Inadequacy or incompleteness.
"The nutritional deficiency in his diet led to severe health problems."
In plain English: A deficiency is when you don't have enough of something that you need to be healthy or successful.
"The doctor warned him that his diet had a deficiency in vitamin D."
The word deficiency comes from combining the adjective deficient with the suffix -cy to form a noun meaning a state of lacking something. It entered English through French and shares its roots with the Latin term dēficientia.