Moments of the year when the sun reaches it's southernmost or northernmost position. There are two Solstices a year: The Summer solstice is the longest day of the year and marks the beginning of the summer. Similarly, the Winter solstice is the shortest day of the year, marking the beginning of winter.
The dates of the Winter Solstice and Summer Solstice depend of course on the hemisphere of Earth.
One solstice occurs on June 20,21 or 22, which may be referred to as the June Solstice or the Northern Solstice. It is the Summer Solstice in the Northern Hemisphere and the Winter Solstice in the Southern Hemisphere.
The other solstice occurs on December 21 or 22, which may be referred to as the December Solstice or Southern Solstice. It is the Summer Solstice in the Southern Hemisphere and the Winter Solstice in the Northern Hemisphere.
See also: Equinox