We don't have synonyms for pentagramme yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are pentagram, centigramme and pentagrams.
(n) :
(rare, archaic) Alternative form of pentagram. [The shape of a five-pointed star constructed of five intersecting lines meeting at the vertices, such that a central pentagon and five surrounding isosceles triangles are formed; often with magical connotations; a 5/2 (or 5/3) star polygon.]
(n) :
(rare, archaic) Alternative form of pentagram. [The shape of a five-pointed star constructed of five intersecting lines meeting at the vertices, such that a central pentagon and five surrounding isosceles triangles are formed; often with magical connotations; a 5/2 (or 5/3) star polygon.]