Refers to a two-wheeled motor vehicle. Slightly more formal or technical than バイク, which is more common in everyday speech.
He commutes by motorcycle.
単車の免許を取るには教習所に通わなければならない。
To get a motorcycle license, you have to attend a driving school.
motorcycle decorated with anime characters
Compound of 単 (single) and 車 (vehicle), literally 'single vehicle', referring to a two-wheeled vehicle as opposed to a sidecar-equipped motorcycle or a car.