What is the meaning of ''-'' in dx2-y2 orbital?

Dear Atri,
It does not designates subtraction since subtraction of planes is not possible,
as we represent x-y plane .....the same is for dx2-y2 orbital depicting x2 and y2 plane.
Regards.

hiphen

Crystal field theory is a standard topic in current U.S.textbooks for undergraduate general chemistry. The key concept that guides this theory is the spatial orientation of thedifferent d orbitals: while dxy, dyz, and dxz lie between the axes,dx2−y2 and dz2 are aligned along in the x, y, and z direction.The interaction of ligands with the two different sets of orbitals induces a splitting in the d-orbital energy levels, andthis splitting gives rise to the color of transition-metal complexes. d Orbitals are introduced early in many textbooks,usually in the chapter about atomic structure. Many textbooks(for example, 1–3) present a diagram of the orbital shapesand their labels, without any reference to the meaning of thelabels. Students who study crystal field theory need to knowthe shapes of the five d orbitals, the labels of the five d orbitals, and how to match the labels to the shapes. This is a lotof data to memorize! When faced with this quantity of unrelated data, most students learn it by rote memorization.

Table 1. Polar and Cartesian Representationsof the Angular Part of d-Type Real Hydrogen-likeWave FunctionsOrbitalPolarRepresentationCartesian Representation ona Unit Sphere (r = 1)dxysin2θ sin2? 2xydx2−y2 sin2θ cos2? x2− y2dxzcosθ sinθ cos? xzdyzcosθ sinθ sin? yzdz2 3cos2θ − 1 2z2− (x2+ y2)

It does not designates subtraction since subtraction of planes is not possible,
as we represent x-y plane .....the same is for dx2-y2 orbital depicting x2 and y2 plane