any of several usually small short-bodied breeds originally trained to hunt animals living underground
"The energetic terrier dug into the dirt, determined to flush out the fox hiding in its burrow."
A dog from a group of small, lively breeds, originally bred for the hunting of burrowing prey such as rats, rabbits, foxes, and even otters; this original function is reflected in some of their names (e.g. rat terrier).
"He used a wooden terrier to drill holes in the old log cabin floorboards."
An auger or borer.
In plain English: A terrier is any of several types of small, energetic dogs originally bred to hunt and kill vermin underground.
"The energetic terrier dug under the fence to chase after the squirrel."
Usage: A terrier is any member of a family of small, energetic dogs historically developed to hunt vermin like rats and foxes. While often confused with other working breeds due to their similar size or coat types, the term specifically refers to these burrowing specialists rather than general-purpose hunting dogs.
The word "terrier" comes from the Middle French phrase chien terrier, which literally meant a "dog of the earth." It entered English to describe dogs bred for hunting vermin by digging into burrows.