deficiency vs lack

deficiency and lack both are nouns.

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

As nouns, deficiency and lack are synonyms defined as:

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

Other synonyms of deficiency include want.

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

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