Zelator is the second grade of the Hermetic Order of the Golden Dawn's First Order, numbered 1=10 in the order's grade notation, which pairs a grade number with a corresponding sphere on the Tree of Life. It follows Neophyte and precedes Theoricus.
The Golden Dawn's grade structure was not built from nothing when the order was founded in London in 1888 by William Wynn Westcott, Samuel Liddell MacGregor Mathers and William Robert Woodman: it was adapted from the earlier Societas Rosicruciana in Anglia, a Masonic-Rosicrucian body founded roughly two decades before, whose own nine grade sequence supplied the names and numbering the Golden Dawn's First Order grades still carry. This borrowing is documented in the scholarly history of the order's founding, including the surviving cipher manuscripts the founders used as their starting material.
The five First Order grades, Zelator among them, are grouped together in the order's overall three tier structure, First, Second and Third Order, as the outer portion of the system: a candidate must complete all five, then pass the bridging Portal grade, before reaching the Second Order that begins at Adeptus Minor. The First Order's teaching is generally understood, in the published scholarship on the order, to correspond to the four classical elements and their synthesis, a structural theme reflected in the five grade sequence itself.