A road, path or tunnel built to pass underneath another road or a railway, so that the two do not cross each other. There are two everyday kinds and it helps to keep them apart. One is for cars: a lane that dips down and runs beneath a busy junction — Taipei drivers meet these all the time. The other is for people on foot, a short tunnel with steps or a ramp at each end, so that you can get to the other side of a big road without walking across it; British speakers call this a 'subway' too, which is a real trap for learners because in American English 'subway' means the underground train. If you want to be understood everywhere, 'underpass' is the safe word, or 'pedestrian underpass' when it is the walking kind. Its opposite is 'overpass' (a road carried above another road) — and in British English an 'overpass' for people is usually called a 'footbridge' or 'flyover' for cars. Underpasses have a mild reputation in English-speaking countries for feeling dark and unsafe at night, so you will often see them described as grim, damp, or covered in graffiti. It is a countable noun: an underpass, two underpasses.