Pentagram

A pentagram is a five-pointed star, which has many symbolic meanings in magic. It was originally a symbol of good magic, before the forces of evil stole it giving it negative connotations in the public eye. ("Apocalypse Not")

A spiritual nexus is the point in the center of a geographical pentagram created by the five spiritual elements (Earth, Fire, Water, Wood, Metal). The Halliwell manor was located at the center of a spiritual nexus, making it a battleground for good and evil. The Charmed Ones realized this in 1999 when the Shadow or 'Woogyman' tried to take over than manor. ("Is There a Woogy in the House?")

While battling the Horsemen of the Apocalypse, the Charmed Ones inadvertently formed a Pentagram with three of their members with Prue and War in the center. This combined with each of their magics trying to destroy the others formed a vortex that trapped Prue and the Horseman of War in another plane. Both of them were rescued from the plane when they formed the pentagram again. ("Apocalypse Not")