Slang is the use of informal words and expressions that are not considered standard in the speaker's language or dialect but are considered more acceptable when used socially. Slang is often used as a euphemism and may use informal lexicon to identify with one's peers.

Few linguists have endeavoured to clearly define what constitutes as slang. Attempting to remedy this, Bethany K. Dumas and Jonathan Lighter argue that an expression should be considered "true slang" if it meets at least two of the following criteria:

Slang is different from jargon, which is the technical vocabulary of a particular profession, and which meets only the second of the criteria given above. Jargon, like many examples of slang, may be used to exclude non–group members from the conversation, but in general has the function of allowing its users to talk precisely about technical issues in any given field.^{[citation needed]}

Slang can be regional (that is, used only in a particular territory), but slang terms are often particular instead to a certain subculture, such as music or video gaming. Nevertheless, slang expressions can spread outside their original areas to become commonly used, like "cool" and "jive." While some words eventually lose their status as slang (the word "mob", for example, began as a shortening of Latin mobile vulgus), others continue to be considered as such by most speakers. When slang spreads beyond the group or subculture that originally used it, its original users often replace it with other, less-recognised terms to maintain group identity.