The descent of the Spirit into matter as symbolized by the Ray of Creation and the gradual condensation or crystallization of matter in ever increasing densities. Do not confuse this term with devolution, the simplification of matter and energy that always follows any process of evolution or complication of energy. Through involution, the Spirit becomes "involved" with nature.
"We have already passed through involution (descent of the Spirit into matter). We also suffered horribly in evolution (process of complication of the energy). Now, a complete revolution (dissolution of the “I”) is urgent." - Samael Aun Weor, The Aquarian Message
Involution is the involvement of the Ain Soph (limitless) into matter; this process is performed by the Monads that emerge from out its bosom and descend through its light-ray (Ain Soph Aur) into the universe (adverse of the unity). Once the involution ends, the mechanical process of evolution and devolution begins.
Involution is the process by which the Absolute manifests matter, energy, and cognizance within the universe; it is the process by which diversity emerges from unity.
Involution is the necessary decent of the Noumenon into the Phenomena, that is, the necessary descent of that which is unknowable to itself, in order to become knowable to itself.
Involution is the process of how part of the Absolute becomes self-limited by veiling its cognizance from itself by stages into the complication of the matter and energy until it assumes an illusory appearance in the seven cosmos.
Involution is the process where the Absolute becomes infinite so that its spiritual properties can awaken into infinite possibilities, for the only purpose of knowing itself.
Involution is the descent of the Monads into the universe in order to devote themselves to the attainment of self-cognizance through the building of energy-matter-cognizant vessels, so that the noumenon can manifests its properties.
Through involution, the Ain Soph (limitless) gradually carries its life into denser and denser cosmic matters to the lowest point of materiality, in order to manifest its properties; from that point, in order to victoriously re-ascend towards its original point of departure, its life must gradually revolve by means of its cognizance and will, thus succeeding in the objective of its existence, during which the Monad develops self-cognizance.