for any curve in spacetime. This terminology causes confusions. More properly, a world line is a curve in spacetime that traces out the (time) history of a particle, observer or small object. One usually uses the proper time of an object or an observer as the curve parameter along the world line.

Trivial examples of spacetime curves

A curve that consists of a horizontal line segment (a line at constant coordinate time), may represent a rod in spacetime and would not be a world line

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