The Solar Charm is a spell that manifests powerful solar energy that can be used to attack an opponent.
The energy from the spell can be released in multiple ways as seen by Mel Vera and several of the Elders. When Mel casts the spell, the energy manifests in a glowing orb that she is able to fire at her opponent. The orb explodes into a blast of blue light and releases a sonic boom that is capable of vanquishing or severely harming those in its path. When the Elders cast the spell, they manifested the energy into beams of white and blue light.
In "Sweet Tooth", Mel used this spell twice. First, in a training simulation and later when battling the Harbinger of Hell. When used against the Harbinger, Macy Vaughn was caught in the blast which resulted in her near death.
Solaris means solar while incantatio means charm or enchantment. It can be roughly translated to "Solar Charm".