proper length

Just as proper time is the time a clock reads in its own rest frame, proper length is the length of an object measured in the frame where the object sits still. It is the 'real', tape-measure length you would get by walking up to the object at rest and measuring it at your leisure — the length stamped on the spaceship's blueprints. Every other frame, seeing the object move, will measure it as shorter.

Precisely: the proper length L_0 of an object is its length measured in its own rest frame. In any frame where it moves at speed v along its length, the measured length is contracted to L = L_0 / gamma, with gamma = 1 / sqrt(1 - v^2/c^2) the Lorentz factor. Because gamma is at least 1, the proper length is the largest length any inertial observer measures; all moving frames see a shrunken version. Proper length can also mean the rest-frame distance between two points, such as the distance between two stars measured by someone at rest relative to them.

Proper length is the natural partner of proper time, and keeping the two straight prevents most beginner confusion. When you read that 'the object is 100 m long', ask: in whose frame? If it is the object's own frame, that is the proper length; any other observer will record less. There is no single true length any more than there is a single true 'now' — only the rest-frame length, which is special because it is the biggest and the one the object 'agrees' with.