The description of the word you requested from the astronomical dictionary is given below.
A red giant or red giant star is a star that is much bigger and cooler and redder than the Sun is. Red giants have diameters up to a few hundred times as large as that of the Sun. In about 5 thousand million years, the Sun will turn into a red giant, when the hydrogen in the center of the Sun starts to run out. The diameter of the Sun will then grow to about 40 times the size it has today.