A culvert is a drain or pipe that allows water to flow under a road, railroad, trail, or similar obstruction. Culverts differ from bridges mainly in size and construction. Culverts are generally smaller than bridges, ranging from 0.3-metre (1 ft) pipes to larger reinforced concrete structures.

For legal purposes, US structures with total spans under 6 metres (20 ft) are considered culverts, and longer structures are bridges. When boxes or pipes are placed side-by-side to create a width of greater than twenty feet, the culvert is defined as a bridge in the United States. This is a requirement of the federal bridge inspection standards and ensures that the culvert is inspected on a regular basis.

Transportation routes cross over waterways using either a bridge or a culvert. Culverts come in many sizes and shapes. Shapes may include round, elliptical, flat-bottomed, pear-shaped, and box. Culverts may be made of concrete, galvanized steel, aluminum, or plastic, typically high density polyethylene.