Loci (singular locus) are where spirit energy bleeds into the physical world. A weak spot in the Gauntlet and Essence flows more freely. It’s only in the presence of a locus that a werewolf can overcome the great burden of being Forsaken and enter the spirit world once more.
Loci are focal points of spiritual energy in the physical world with particular flavors, or resonances. They can result from The Shadow being influenced by human emotion or the active tearing by The Hosts.