Sri Aurobindo uses the term "central being" to refer to what might in most esoteric traditions, be called the "Higher Self"; the underlying or overshadowing spiritual principle around which the rest of being is organised, and which guides and directs the person. It refers to the transcendent and eternal spirit, as opposed to the incarnate and evolving Soul, which he calls the Psychic Being, although sometimes it refers to both of them together as the essential spiritual core of the being. These definitions are found in Letters on Yoga vol.I under "Planes and Parts of Being" (pp.265ff in the 3rd ed.)
The Central Being "presides over the different births one after the other but is itself unborm" (ibid p.269). This transcendent Central Being or Spirit is also designated the Jiva or Jivatman, although the meaning of these terms in Sri Aurobindo's philosophy differs greatly from that of much of conventional Vedanta (especially Advaita Vedanta)
This concept of a higher spiritual Guide or individual Divine is found in many Western esoteric teachings - the Angel of Valentinian, the Fravashi of ancient Iranian thought (Zervanism), the daimon paedros referred to by Plotinus (Enneads 3:iv) into whose care we are given, and who is the guide of the soul (= ego / conscious self?) throughout life and beyond death, the Holy Guardian Angel of Golden Dawn and OTO Hermeticism. But whereas these teachings refer to a single principle, Sri Aurobindo is careful to distinguish that two distinct principles are being described here: one transcendent and one evolutionary (and sometimes three principles are referred to). Even so he still tends to use the term "central being" ambiguously to two different (but related) things. From Letters on Yoga:
"The phrase "central being" in our yoga is usually applied to the portion of the Divine in us which supports all the rest and survives through death and birth. This central being has two forms
above, it is Jivatman, our true being, of which we become aware when the higher self-knowledge comes,
below, it is the psychic being which stands behind mind, body and life.
The Jivatman is above the manifestation in life and presides over it;
the psychic being stands behind the manifestation in life and supports it."
"The central being (Jivatma) is that which is not born, does not evolve, but presides over all the individual manifestation.
The psychic is its projection here - for the psychic being is in the evolution and from within supports our whole evolution; it receives the essence of all experience and by that develops the personality Godward."
These two principles each have very different qualities and modes of consciousness
"The natural attitude of the psychic being is to feel itself as the Child, the Son of God, the Bhakta; it is a portion of the Divine, one in essence, but in the dynamics of the manifestation there is always even in identity a difference.
The Jivatman, on the contrary, lives in the essence and can merge itself in identity with the Divine; but it too, the moment it presides over the dynamics of the manifestation, knows itself as one centre of the multiple Divine, not as the Parameshwara...."
As regards realisation
"The Jiva is realised as the individual Self, Atman, the central being above the Nature, calm, untouched by the movements of Nature, but sup- porting their evolution though not involved in it. Through this realisation silence, freedom, wideness, mastery, purity, a sense of universality in the individual as one centre of this divine universality become the normal experience.
The psychic is realised as the Purusha behind the heart. It is not universalised like the Jivatman, but is the individual soul supporting from its place behind the heart-centre the mental, vital, physical, psychic evolution of the being in Nature. Its realisation brings bhakti, self-giving, surrender, turning of all the movements Godward, discrimination and choice of all that belongs to the Divine Truth, Good, Beauty, rejection of all that is false, evil, ugly, discordant, union through love and sympathy with all existence, openness to the Truth of the Self and the Divine."