Mich. Comp. Laws § 257.59: “Semi-trailer” defined.

Sec. 59.

"Semi-trailer" means every vehicle with or without motive power, other than a pole-trailer, designed for carrying persons or property and for being drawn by a motor vehicle and so constructed that some part of its weight and that of its load rests upon or is carried by another vehicle. Semi-trailer does not include any implement of husbandry.