Citation is a reference to a published or unpublished source (not always the original source).
More precisely, a citation is an abbreviated alphanumeric expression (e.g. [Smith 2020]) embedded in the body of an intellectual work that denotes an entry in the bibliographic references section of the work for the purpose of acknowledging the relevance of the works of others to the topic of discussion at the spot where the citation appears.
Key elements of a citation include:
1. Author name(s)
2. Title of work
3. Date of publication
4. Page numbers (where applicable)