The magnitude of the dimmest object which can be seen using a telescope of a given aperture or with your naked eye.
The faintest stellar object visible. A telescope's limiting magnitude is directly proportional to its light-gathering power. The larger a telescope's aperture, the fainter its limiting magnitude.
The magnitude of the dimmest object that can be seen through any optical instrument (including the eye). The limiting magnitude of an instrument varies with light conditions.