countable; denotes a vehicle or similar designed for one person
The race was limited to lightweight single-seaters.
Гонка была ограничена лёгкими одноместниками.
attributive relational adjective meaning 'designed for one occupant'; not gradable
He flew a single-seater fighter plane during the war.
Во время войны он пилотировал одноместный истребитель.