Definition - A word or phrase that is only used in a particular geographical area and which is not commonly used by speakers from outside of that area.

Example -
The word skookum is a Chinook jargon word that is a localism in BC and the Yukon. It basically denotes "that which is good"; e.g, "That was a skookum party last night. I passed out by the railroad tracks."

Oxford English Dictionary -
The term's first citation is from 1823:
"Suffolk Words and Phrases, or an attempt to collect the Lingual Localisms of that County."
(E. Moor (title) )

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