A sacrificial knife dedicated to God. The Smith family is known to have at least one athame, the Mars Dagger. This was a silver dagger with the symbol of Mars that belonged to Hyrum Smith (See Early Mormonism and the Magical World View, Quinn, Figure 43). Before Christ, the athame was used to sacrifice animals.
It represents judgment, fire, and the Holy Spirit. With the exception of Levites, modern athames should be made with a covenant not to be used to shed blood and are desecrated if blood touches them.
In temples of the Church of Jesus Christ of Latter-day Saints, for example, a finger is used in place of an athame for cutting the prayer circle to add someone entering after the circle has been closed.
See 1 Moses 31:19, Book of Remembrance 33:42.