A trooper is a specific kind of police officer, typically one who patrols highways and enforces traffic laws across a state. State troopers drive marked patrol cars and help keep roads safe by watching for dangerous driving, responding to accidents, and assisting stranded motorists. You might see a trooper's vehicle parked along the interstate, monitoring traffic, or you might wave to one who's helping someone with a flat tire.

In some places, mounted police officers are still called troopers. Some police departments, like the Massachusetts State Police, call all their officers troopers, regardless of their specific assignment.

People also use trooper to describe someone who handles difficulty with courage and determination. If your friend twists her ankle during a hike but keeps going without complaining, you might say she's “a real trooper.” When someone says you were “a good trooper” about getting a shot at the doctor's office, they mean you stayed brave and didn't make a fuss.