cor·ner·stone
/ˈkôrnərˌstōn/
noun
noun: cornerstone; plural noun: cornerstones; noun: corner-stone; plural noun: corner-stones
1. an important quality or feature on which a particular thing depends or is based.
2. a stone that forms the base of a corner of a building, joining two walls.
3. A clue for a geocache hidden in a flowerbed at Lone Star Lakes Park