Usus (from Latin: usus — usage; long-established rule, practice, custom) — widely accepted usage of linguistic units (words, idioms, forms).
On one hand, it is set off occasional usage and, from the other hand, also language norm. Sometimes the term usus implies particular norms differing from literary norm (e.g. professional or social norms). Usus significantly differs from language norm. Also, being closely related to it, it might also even become one or be registered in dictionaries.
Usus is one of the crucial terms in researches of Danish linguists Otto Jespersen and Louis Hjelmslev.
References
Usus Wikipedia(Text) CC BY-SA