a Bragg plane
/ Bragg -> BRAG /
In Bragg's canyon-of-cliffs picture, the cliff walls are the Bragg planes: a family of parallel, equally spaced planes of atoms that together act as the effective mirror for one reflection. Every reflection you can record belongs to one such family, named by its Miller indices (hkl) and characterised by a single spacing d_hkl. When people say a crystal reflects off its (111) planes, the (111) family is the Bragg plane set doing the reflecting.
It helps to see why a family, not a single plane, is what matters. A lone plane of atoms would scatter weakly and at any angle; it is the stack of many identical planes, all spaced d apart, that enforces the sharp angular selection. Only when the path difference between neighbouring planes, 2 d sin theta, equals a whole number of wavelengths do all the planes in the stack reinforce; miss the angle slightly and the deep planes fall out of step and cancel the shallow ones. The more planes contribute, the sharper the surviving beam.
There is a second, more advanced meaning worth flagging honestly. In reciprocal space, a Bragg plane is the flat plane that perpendicularly bisects a reciprocal lattice vector g_hkl; a wave whose wavevector ends on that plane satisfies the diffraction condition, and these bisecting planes are exactly the walls of the Brillouin zones used in solid-state physics. Both meanings describe the same physics from two viewpoints — the reflecting family in real space, and its perpendicular-bisector plane in reciprocal space.
Aluminium (FCC, a = 4.05 angstrom) reflects strongly off its {111} Bragg planes, spaced d = 4.05 / sqrt(3) = 2.34 angstrom. Because {111} contains eight symmetry-equivalent planes with the same d, they all diffract at the same angle and pile into one strong powder line.
A Bragg 'plane' is really a family sharing one d-spacing; symmetry-equivalent members overlap in a powder pattern.
A single atomic plane does not make a reflection; a whole family of parallel planes spaced d apart does. And only in the cubic system does the direction [hkl] point along the normal of its Bragg plane (hkl) — in other systems use g_hkl for the normal.