A pointed weight, designed to be attached to and dangled from a string, to determine a true vertical line, or to orient two points perfectly vertically in space (that is, the point the string is dangled from and the point the tip of the plumb bob points to, e.g. a transit level on a tripod and the benchmark on the ground you're taking measurements from).

Like plumbing, the name "plumb bob" (and the adjective "plumb", meaning perfectly vertical) comes from the Latin word for lead, plumbum.