lack vs deficiency

lack and deficiency both are nouns.

Word	Noun	Adjective	Verb	Adverb
lack	Yes	No	Yes	No
deficiency	Yes	No	No	No

As nouns, lack and deficiency are synonyms defined as:

lack and deficiency: the state of needing something that is absent or unavailable

Other synonyms of lack include want.

lack (noun)	deficiency (noun)
the state of needing something that is absent or unavailable	lack of an adequate quantity or number
	the state of needing something that is absent or unavailable

lack (verb)	deficiency (verb)
be without
To be insufficiently prepared