The Golden Dawn's system of initiation is a ladder, and the rungs are mapped one-to-one onto the sephiroth of the Tree of Life, counted upward from the bottom. The notation is two numbers, the first counting up from the entry grade and the second counting down from the top, so that the ascent runs 1=10 to 10=1. Knowing the ladder makes sense of a good deal of later occult vocabulary that uses its rank-names loosely.
The First or Outer Order taught the theory. Neophyte (0=0) stood outside the Tree; then Zelator (1=10, Malkuth), Theoricus (2=9, Yesod), Practicus (3=8, Hod) and Philosophus (4=7, Netzach).
The Second or Inner Order, the R.R. et A.C., was where the practical magic actually began, entered only after a portal grade. Its grades were Adeptus Minor (5=6, Tiphareth), Adeptus Major (6=5, Geburah) and Adeptus Exemptus (7=4, Chesed). In practice almost all of the celebrated members never advanced beyond Adeptus Minor; the higher adept grades were largely notional.
The Third Order lay across the Abyss and was held to be occupied by the 'Secret Chiefs', not by living initiates: Magister Templi (8=3, Binah), Magus (9=2, Chokmah) and Ipsissimus (10=1, Kether). In the original order these were unattainable by definition.
Crowley's later A.'.A.'. took the same ladder but treated the grades above the Abyss as goals an aspirant could actually claim to reach, and claimed them himself, which is the source of the modern use of Magus and Ipsissimus as titles a person might hold. The general terms Initiate, Neophyte and Adept have since drifted out of the system entirely and are used across modern occultism for, respectively, anyone admitted, a beginner, and an experienced practitioner, with none of the precise placement the ladder gave them.