A standing stone is a monolith, or possibly part of some larger group. Stone circles are a common prehistoric feature of Britain. Standing stones are upright, and made of almost any kind of inidigenous rock. There also recumbent stones, as seen at Stonehenge and the Rollright Stones. The Avenue at Avebury is a double row of standing stones, and there are other examples in all corners of the UK.

Other references:
In the Forgotten Realms AD&D campaign setting, the Standing Stone is a monument set up in the year 1 to mark a treaty between elves and humans.
The Standing Stone is an album of sub-standard pseudo-classical music by Sir Paul McCartney.
Standing Stones is a Magic: the Gathering card.