Reference

thumb|A post (tweet) on the social network X (social network)|X (Twitter) with several hashtags colored in blue text. A hashtag is a metadata tag operator that is prefaced by the hash symbol, #. On social media, hashtags are used on microblogging and photo-sharing services–especially Twitter and Tumblr–as a form of user-generated tagging that enables cross-referencing of content by topic or theme. For example, a search within Instagram for the hashtag #flowers returns all posts that have been tagged with that term. After the initial hash symbol, a hashtag may include letters, numerals or other

thumb|xkcd webcomic titled "[[Wikipedian Protester". The sign says: "".]] A citation is a reference to a 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.

metadata used for classifications or adding of informations

In logic, a reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to refer to the second object. It is called a name for the second object. The next object, the one to which the first object refers, is called the referent of the first object. A name is usually a phrase or expression, or some other symbolic representation. Its referent may be anything – a material object, a person, an event, an activity, or an abstract concept.

text placed at the bottom of a page or at the end of a chapter

An annotation is extra information associated with a particular point in a document or other piece of information. It can be a note that includes a comment or explanation. Annotations are sometimes presented in the margin of book pages. For annotations of different digital media, see web annotation and text annotation.

public service counter in a library

common format for concordance lines

open XML-based language

service by which a library reference service is conducted online, and the reference transaction is a computer-mediated communication

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