What is the term for the length of an object measured in the reference frame in which the object is at rest?

The term for the length of an object measured in the reference frame in which the object is at rest is known as proper length. This concept is fundamental in the context of relativity, where measurements of length can vary depending on the relative motion of the observer and the object being observed.

When an object is at rest in its own frame of reference, the length measured is not subject to any relativistic effects, such as length contraction, which occurs when the object is moving relative to an observer. Therefore, the measurement taken in the object's rest frame is considered the most accurate representation of its length, free from the influences that arise when the object is in motion.

While other terms like "rest length" may be used colloquially, in the context of physics, "proper length" is the more precise term that captures the essential idea. This term is well-defined within the framework of special relativity, emphasizing that the length remains invariant for observers measuring in the object's rest frame.