The Pentacle / Disk is one of the magical weapons, or elemental tools, of Western ceremonial magic, the set of implements the magician consecrates and uses in ritual. It stands for the body, the material and the manifest; it grounds and gives form.
Tool and element
In the four-fold scheme it is attributed to the element of Earth. The four elemental weapons, Wand, Cup, Sword and Pentacle, correspond to the four suits of the tarot's Minor Arcana and to the four points of the ritual circle, a scheme drawn together in the Hermetic Order of the Golden Dawn from 1888 out of older Solomonic and grimoire material.
A word with two meanings
In the older grimoire tradition, including the Key of Solomon, "pentacle" more often names an inscribed talisman or seal, a disc or parchment bearing a divine name, sigil or geometric figure and worn or displayed for a specific purpose, rather than a standing elemental tool kept on the altar; the Golden Dawn's fourth elemental weapon, a plain engraved disk representing Earth itself, is a narrower, later repurposing of that older word for a new, more structural role within its own fourfold scheme. The two uses now coexist in modern practice without much confusion, since context, a single ritual disk on an altar versus a specific talisman made for one working, usually makes clear which is meant. The weapon is described here as an element of that documented tradition of ceremonial magic, a body of practice studied historically rather than endorsed.
Sources The Magicians of the Golden Dawn: A Documentary History of a Magical Order 1887-1923Ellic Howe with Grimoires: A History of Magic BooksOwen Davies