a truck consisting of a tractor and trailer together
"The semi was hauling a heavy load of construction materials down the highway at dawn."
a trailer having wheels only in the rear; the front is supported by the towing vehicle
"The semi detached from our truck, leaving its front end hanging freely before we could secure it again."
A semi-detached house.
"We decided to paint our semi-detached house a bright blue last weekend."
In plain English: A semi is a large truck with two trailers attached to it used for hauling heavy loads.
"The semi was parked in the driveway waiting to be loaded with furniture."
Derived from the Latin word semis, meaning "half," the term entered English primarily as a prefix to indicate partialness before it began functioning independently as a noun. This shift allowed speakers to refer directly to something that is only partially true or complete, such as in phrases like "a semi-final."