The apparent magnitude (m) of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value (i.e. inverse relation). The Sun, at apparent magnitude of 27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere. Furthermore, the magnitude scale is logarithmic; a difference of one in magnitude corresponds to a change in brightness by a factor of 100, or about 2.512.

Based on this passage, the apparent visual magnitude of a star is a measure of the star's? Let's think fast. Stream of consciousness:
The relevant sentence in the passage is: The apparent magnitude (m) of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. So the final answer is brightness as seen by an observer on Earth.