A concept originating in Hinduism, an avatar (/ˈævəˌtɑr, ˌævəˈtɑr/;[1] Hindustani: [əʋˈt̪aːr] from Sanskrit अवतार avatāra "descent") is a deliberate descent of a deity to Earth, or a descent of the Supreme Being (e.g., Vishnu for Vaishnavites), and is mostly translated into English as "incarnation", but more accurately as "appearance" or "manifestation".

Within the Cthulhu Mythos, an avatar generally holds the same meaning. An Elder God, Great Old One, or an Outer God may decide to appear in a physical or spectral manifestation to humans or their inhuman brethren. The reasons are varied for which an Eldrich entity might need to manifest an avatar. Most deities and entities of power in the Cthulhu Mythos do not or cannot exist in mundane, three-dimensional space. Alternatively, they may be otherwise separated by vast distances of space, sequestration in a secret location, or by virtue of living in another universe or dimension. Manifesting an avatar is often the only way such a being can communicate and interact with its worshippers or have physical agency in a particular location.

An avatar is exponentially weaker but requires a considerable effort of will on the part of its deity to manifest. Avatars may be capable of being harmed, killed or confounded by mortals, though to a cosmic Eldrich entity, such interference is less than the buzz of a fly.