What’s the Story Behind the Name ‘Semi Truck’?

Semi trucks go by various names, such as tractor-trailer, 18-wheeler, and the casual big rig. Some people even omit the truck and refer to them simply as semis. Given their enormous size compared to most vehicles, calling them a term that suggests half truck or partial truck can seem a bit off.

However, as International Used Truck Centers points out, the term semi truck is a shorthand for semi-trailer truck (or sometimes semi-tractor-trailer). A typical trailer can stand on its own, attached to the front vehicle by a drawbar or similar mechanism. In contrast, a semi-trailer has no front wheels and is supported by the rear wheels of a tractor, which carries part of its weight.

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There’s quite a bit of variation in how people describe and define these vehicles. Sometimes, the tractor is simply called a 'tractor,' likely because it’s one of the two components that form a complete semi-tractor-trailer. It could also be for clarification, as truck might refer to a pickup truck (or other types of trucks), and tractor could refer to a farm tractor (or another type of tractor).

In everyday conversation, if someone mentions a semi or semi truck—perhaps to wonder why they often have spikes on their wheels—it’s likely they’re referring to the entire big rig, not just the front or back part. However, if truck drivers are part of the discussion (who are probably more familiar with the specific terms), it might be worth asking for further clarification.