A p trap is a curved, "U" or "J" shaped section of the drain pipe that holds a water seal to prevent gasses from a septic/sewer from entering the home through a fixtures water drain.
A trap is a bend in the pipework that keeps a constant level of water in a drainage or waste system. This stops smells and fumes coming back up the drains and through your plug hole or toilet pan....
When the pipe goes down, and back up, and back down again, the first "up-curve" is called the trap because when the water goes down the drain, not all of it goes back up and down again (thanks to...