Causal plane is a term used in Neo-Theosophy, some contemporary Vedanta, the New Age,, and sometimes Occultism, to describe a high spiritual plane of existence. However there is great variation between the different definitions.
Neo-theosophy
The Neo-theosophy of Annie Besant and C. W. Leadbeater replaced Blavatsky's "Higher Manas" principle with the "Higher Mental", "Abstract Mind", or Causal Body. The equivalent cosmic plane is the Causal Plane. A detailed description of the Causal Plane, along with the Causal Body, is provided by A. E. Powell, who has compiled information in the works of Besant and Leadbeater in a series of books on each of the subtle bodies. The Neo-theosphical concept of Causal Plane proved very influential both in India and in the New Age, via Alice Bailey.
incorporates Theosophical ideas into the traditional Hindu Vedic cosmology, when he describes three worlds of existence as Bhuloka or physical plane, Antarloka or the subtle or astral plane, and Sivaloka or the causal plane or Karanaloka, the world of the Gods and highly developed souls. About this latter he says
The causal plane is the world of light and blessedness, the highest of heavenly regions, extolled in the scriptures of all faiths. It is the foundation of existence, the source of visions, the point of conception, the apex of creation. The causal plane is the abode of Lord Siva and His entourage of Mahadevas and other highly evolved souls who exist in their own self-effulgent form—radiant bodies of centillions of quantum light particles.
Although Neo-theosophy presents the sequence Physical-Astral-Mental-Causal, in the jargon of some astral travelling teachings, the causal plane is the one just above the astral plane. According to the Astral Voyage website this is the region where memories and karmic patterns are stored, and where a practitioner can access information regarding the past and future.
The complex metaphysics of the Channelled entity Michael includes a description of the universe in terms of seven planes. These are the Physical plane, the Astral plane, the Causal plane, the Akashic plane, the Mental plane, the Messianic plane, and the Buddhaic plane. Here the Causal plane is the third plane of creation, and its medium is concrete intellectual energy. In The Michael Teachings, this is Michael's plane of existence.