Beings of unknown size and intent which are believed to exist entirely in an energy or electronic state. A sampling of radio waves, no matter how small, causes cataclysmic damage to their technology, essentially killing them off.
Among other unknowns, the entities exist as one with their defenses. If an energy beam is transmitted for the express purpose of annihilating an enemy, the consciousness of the entity driving that force travels with it, almost like a suicide bomber. The species can transmit themselves both ways through an active Stargate, and in a matter of moments, inflict damage to conventional Earth equipment.
The entities are self-aware and protective of their species, even to the degree of self-sacrifice. They are able to download themselves into Earth devices, such as the S.G.C. mainframe, as well as portable devices as small as M.A.L.P.s -- and even the human brain, which one entity did when it took over Samantha Carter.
FIRST APPEARED - Entity
Entity - Stargate Command inadvertantly causes great damage to the energy-based entities when they send a M.A.L.P. probe, prompting an attack of retaliation.