What term is used to describe a kind of language occurring chiefly in casual and playful speech, often made up of short-lived expressions?

The term that accurately describes a kind of language occurring chiefly in casual and playful speech, characterized by short-lived expressions, is slang. Slang encompasses informal words and phrases that often arise within specific social groups or communities, and they tend to reflect the culture and dynamics of that group. Slang is highly dynamic; it can change rapidly, with some expressions becoming outdated as new ones emerge.

Dialect refers to particular forms of a language that are specific to a region or social group, which is broader and lasts longer than slang. An idiom consists of expressions whose meanings are not deducible from the individual words (e.g., "kick the bucket"). Colloquialism refers to informal language used in everyday conversation, which may include slang but also comprises accepted phrases that are more stable over time compared to slang. Therefore, slang is the most fitting choice for identifying casual, playful language made up of fleeting expressions.