hex sign

Short definition from Wiktionary

Noun

1. 1
   A geometric design usually consisting of a star or flower placed in a circular field, used by the Fancy Dutch on buildings such as barns, often said to be intended as a ward against evil in former times.
2. 2
   A protective symbol used in Heathenry comprising a six-petaled flower or star set within a circle, as a New Age adaptation of the above.