Aspect of "Self" that resides in the higher density levels of one's experience - one's true awareness and God connection. There can be many aspects/layers of Higher Self (call them Soul, Oversoul or Higher Self directly) representing different aspects of consciousness that are being played out throughout time/space, be it earth-human or otherwise. The highest level of Higher Self is like the hub of a wheel - the union or focal point of all these various aspects. It is the animating force within each of us that guides our lower Selves to search for higher understanding and expression, to ultimately reach the state of transformation of becoming an integrated whole. See Multi-dimensional Self.
higher self: another name for your oversoul, or superconscious mind. In the Alice Bailey book, this was referred to as "Soul." It is the self that is the guide and teacher for every incarnated person up to the fourth initiation. At this time the incarnated individual who is really the soul who is incarnated has merged with consciousness of the Higher Self and, hence, a new teacher is made available which is the Mighty I Am Presence, the Monad and/or Spirit itself! Most synonymous term for higher self would be oversoul. Every Oversoul has twelve souls that it is in charge of that make up its Oversouls' group. Every Monad or Mighty I Am Presence has 12 Oversouls or Higher Selves that make up its Monadic Group! Read Dr. Stone's book, The Complete Ascension Manual, for more explanation on this.