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Path length can mean one of several related concepts:
In physics, there are two definitions for "path length." The first is defined as the total distance an object travels. Unlike displacement, which is the total distance an object travels from a starting point, path length is the total distance travelled, regardless of where it travelled. The second is synonymous with wavelength and is used in calculating constructive and destructive interference of waves.
In chemistry, the path length is defined as the distance that light (UV/VIS) travels through a sample in an analytical cell. Typically, a sample cell is made of quartz, glass, or a plastic rhombic cuvette with a volume typically ranging from 0.1 mL to 10 mL or larger used in a spectrophotometer. For the purposes of spectrophotometry (i.e. when making calculations using the Beer-Lambert law) the path length is measured in centimeters (rather than in meters).
In a computer network, the path length is one of many possible router metrics used by a router to help determine the best route among multiple routes to a destination. It consists of the end-to-end hop count from a source to a destination over the network.
More simply, in general computer terminology, it can mean simply the total number of instructions executed from point A to point B in a program - Instruction path length.