Formal/Academic:
* Art. - This is the most common abbreviation, used in bibliographies, footnotes, and other formal writing.
Informal/Casual:
* Art. - Can also be used informally, but other abbreviations are possible depending on the context.
* Article - It's common to just write "Article" in informal settings.
Other abbreviations:
* artcl. - Occasionally used, but less common than "Art."
* a. - Used in some contexts, but can be ambiguous (could also stand for "a" as in "a cat").
It's always best to check the specific style guide or guidelines for your writing to determine the best abbreviation for your context.