The description of the word you requested from the astronomical dictionary is given below.
solstice, plural solstices
A solstice is a moment when the declination of the Sun changes from increasing to decreasing, or from decreasing to increasing. There are two solstices each tropical year: the northern solstice and the southern solstice. The solstices mark midsummer's day and midwinter's day, and the beginning of the astronomical seasons of summer and winter.