1. Celestial event brought about by a combination of the earth's rotation round the sun and the tilt of the angle of the earth's equatorial plane. Over the course of a solar year, the latitude on the earth above which the sun directly shines moves from 23 and a half degrees north (the tropic of cancer) to the same position south (the tropic of capricorn). The solstices on June 21st and December 21st mark, respectively, the northernmost and southernmost points of this apparent journey of the sun across the sky. The equinoxes, on March 21st and September 21st, mark the points when the sun is directly above the equator.
The practical upshot of this is that the angle of the sun in the sky changes, to a greater or lesser extend depending upon latitude; and the range of stars visible changes also.
The variation in the angle of the sun in the sky at different times of the year is the most direct cause of the changes in the seasons. For example, in the northern hemisphere on the December solstice, the sun is low in the sky and it is winter. On the June solstice the sun is high in the sky and it is summer. It also affects the length of the days, which effect increases exponentially with latitude. The solstice is the shortest or the longest day of the year: at the tropics, the summer solstice day is only an hour or two longer than the winter solstice day, whereas at 50 degrees latitude, the difference is about 8 hours.
2. The second definition of a solstice is that it is an excuse for hippies to have a party and worship a variety of mystical things. Evidenced vaguely on the back of a few neolithic buildings that appear to have a solstice marker built in, and a few ancient scriptures that encourage solstice worship wrapped in layers of metaphor, such people believe that the ancients worshipped the solstices above all other events, and that to do so is fundamental to our place in the universe. Some believe it is sacreligious and dishonorable not to do so. As a result, such people are often to be seen hanging around at places like Stonehenge in the middle of the night waiting to watch the solstice sun rise over the stones. What they expect to happen at these places, apart from the sun rising as normal, is a mystery. What is slightly less mysterious is that the summer solstice at Stonehenge attracts many more people than the winter solstice....
Quote from a solstice 'sermon' by the Unitarian Universalists:
“This is the stillness behind motion, when time itself stops; the center which is also the circumference of all. Now darkness triumphs; and yet, gives way and changes into light. We will be awake in the dark. We will call the sun from the womb of night. As we celebrate the solstice we join across time and space all the festivals of light emerging from the dark.”
Sounds very mystical and yoghurty to me. The winter solstice is also of astrological significance, being the time when "saggitarius" moves into "capricorn".
It is generally agreed that Christmas was timed to both continue some of the traditions of, yet overshadow, the pagan festival that once marked the solstice.
The winter solstice is also seen as the birthday of the Buddha, a time to celebrate the cycle of birth and death, the death-day of Osiris, the beginning of the time of the serpent days, the beginning of the month of the birch, the best day of the year for introspection and future planning, and the best day of the year to become a vampyre. Oh and it's the specific end date for the Mayan calendar, when we will reach the end of the world/eschaton/massive shift in global consciousness, or whatever it is. But that's just this year, 2012. Just to be clear, that will not happen every winter solstice. Which is lucky.
There is no doubt that the solstice is an important astronomical event which is a significant marker in the process of the solar year. If you don't live in the tropics, that is. If you live in the tropics the solstice makes precisely bugger all difference to your life, because in the tropics it is hot all the year round and the days are about the same length. Ooh, and more than quarter of the world's population live in the tropics.It is also clear that centuries of religious and ritualistic practice have been ascribed and attributed to the solstices, the scope of which I have only touched on here. Believe what you like, but the only things that are certain about the solstices are described in the first definition.