Wiktionary
slot car
alt. A type of miniature toy car, that is fitted into a slot in a track which it is raced around. n. A type of miniature toy car, that is fitted into a slot in a track which it is raced around.
Wikipedia
Slot car
A slot car (sometimes, slotcar or track car) is a powered miniature auto or other vehicle that is guided by a groove or slot in the track on which it runs. A pin or blade extends from the bottom of the car into the slot. Though some slot cars are used to model highway traffic on scenic layouts, the great majority are used in the competitive hobby of slot car racing or slot racing.