The term involution refers to different things depending on the writer. In some instances it refers to a process that occurs prior to evolution and gives rise to the cosmos, in others an aspect of evolution, and still others a process that follows the completion of evolution in the human form.
In Theosophy, Anthroposophy and Rosicrucianism, involution and evolution are part of a complex sequence of cosmic cycles, called Round. When the universe attains a stage of sufficient density, the individual spirit is able to descend and participate in the evolution. Involution thus refers to the incarnation of spirit in an already established matter, the necessary prerequiste of evolution:
That period of time devoted to the attainment of self-consciousness and the building of the vehicles through which the spirit in man manifests, is called involution. Its purpose is to slowly carry life lower and deeper into denser and denser matter for the building of forms, till the nadir of materiality is reached. From that point, life begins to ascend into higher Worlds. This succeeding period of existence, during which the individual human being develops self-consciousness into divine omniscience, is called "spiritual evolution."
In the cosmology of Surat Shabda Yoga, involution and evolution apply to both the macrocosm, the whole of creation, and the microcosm, the constitution of an individual soul.