The description of the word you requested from the astronomical dictionary is given below.
The elongation of a celestial body is a measure for the distance in the sky between that celestial body and the Sun. The following different definitions are used:
For a celestial body on the ecliptic, both definitions yield the same results.
The elongation cannot exceed 180 degrees by definition. Superior planets can have any elongation up to the maximum 180 degrees, but inferior planets have a maximum elongation that is less than 180°. The maximum elongation of Mercury is about 28°, and that of Venus about 48°.