A Pentagram is a mullet of five points voided and interlaced, commonly used in period heraldry. A Pentacle is a similar symbol, correctly blazoned A mullet voided and interlaced within and conjoined to an annulet.
It can be found in Christian churches like Notre-Dame de Paris where it symbolizes the five points from which Christ bled on the cross. In the poem Sir Gawain and the Green Knight, Sir Gawain bore a pentagram upon his shield and it was also used to represent many other things, e.g. the five knightly virtues.
Pentagrams in the SCA
Both the pentagram and the pentacle are used by modern Pagans as a religious symbol, (usually with a single point upward) representing the Five Elements. The common usage of a reversed pentagram (two points upward) by Satanists had caused this charge to be added to the SCA College of Heralds' list of restricted charges beginning in 1973. However this ruling was overturned in 2009, and both the pentagram and pentacle symbols, both regular and reversed, are now permitted heraldic charges.