It is the apparent diameter of a celestial object, measured in degrees and fractions of degree.
Let us underline the apparent word, since the Sun and the Moon, for example, seen from Earth have an equal angular diameter of approximately half a degree, while their effective diameter is, respectively, 1,392,000 km and 3,476 km. The Sun, therefore, is actually approximately 400 times larger than the Moon; however, it is also 400 times further from the Moon with respect to the Earth and this makes its disk appear identical to the moon.
For very distant celestial objects (galaxies, star clusters, etc.) the angular diameter remains constant and is practically null for the stars.
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