This is any structure that moves water past an obstacle.
Most often, a culvert is a pipe through the Earth. If a drainage ditch runs down the side of the road, and a driveway has to obstruct that ditch, the culvert is the opening (invariably a pipe) through the dirt that is piled in the ditch so a car can drive across it.
But here’s an example of the confusion: Loxley woman dies after her vehicle struck a culvert and flipped over
A woman from Loxley, Alabama died […] when her 2017 Nissan Sentra left the roadway and struck a culvert.
This is not accurate. The woman didn’t strike a “culvert,” she most likely veered off the road into a parallel ditch running beside the road. She would have come to rest in the ditch, but a roadway crossed it, and she struck a formed pile of dirt which caused an impact or sent her airborne.
The “culvert” is technically just the pipe through the dirt to move the water, not the dirt itself, which primarily exists so a car can cross the ditch. She really struck an intersecting driveway or road.
We also tend to associate “culvert” with the ditch itself, when we really mean the perpendicular roadways that periodically cross the ditch (which, as noted, is also wrong).
Culverts don’t even have to be associated with a road. Culverts can exist in the wilderness. They’re a drainage device, not a traffic device.