Definitions for 18-wheelerˈeɪ tinˈʰwi lər, -wi-
Random House Webster's College Dictionary
18-wheelerˈeɪ tinˈʰwi lər, -wi-(n.)
a tractor-trailer equipped with eighteen wheels.
A big rig truck: a semi-trailer plus the truck or tractor pulling it.
Origin: From the total number of wheels: two front steering wheels on the tractor, plus dual wheels on each end of two axles (total of 8) at the rear of the tractor and supporting the front of the trailer, plus dual wheels on two axles at the rear of the trailer.