The term ‘citation’ in interactive text terms refers to the academic tradition of ‘citing a source’, meaning showing where the source of a quote or the basis for an assertion comes from.
With reference to interactive text, it is used as indicating a richer type of link, which has information about the destination of the link added to the source of the link, such as author, publication and so forth.
Broadly, a citation is a reference to a published or unpublished source (not always the original source). More precisely, a citation is an abbreviated alphanumeric expression 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. Generally the combination of both the in-body citation and the bibliographic entry constitutes what is commonly thought of as a citation (whereas bibliographic entries by themselves are not). References to single, machine-readable assertions in electronic scientific articles are known as nanopublications, a form of microattribution.
From Etymology Online:
citation (n.) c. 1300, “summons, written notice to appear,” from Old French citation or directly from Latin citationem (nominative citatio) “a command,” noun of action from past participle stem of citare “to summon, urge, call; put in sudden motion, call forward; rouse, excite” (see cite).
Meaning “passage cited, quotation” is from 1540s. From 1918 as “a mention in an official dispatch.”