In this work, following the mathematics convention, the symbols for the radial, azimuth, and zenith coordinates are taken as r, theta, and phi, respectively. Note that this definition provides a logical extension of the usual polar coordinates notation, with theta remaining the angle in the xy-plane and phi becoming the angle out of that plane. The sole exception to this convention in this work is in spherical harmonics, where the convention used in the physics literature is retained (resulting, it is hoped, in a bit less confusion than a foolish rigorous consistency might engender).
Unfortunately, the convention in which the symbols theta and phi are reversed is also frequently used, especially in physics. The symbol rho is sometimes also used in place of r, and phi and psi instead of phi.