Why Reactance, the opposition to AC offered by inductors and capacitors, is symbolized by X, as in X_L = 2*pi*f*L and X_C = 1/(2*pi*f*C). It combines with resistance R to form impedance: Z = R + jX. Reactance is measured in ohms, but unlike resistance it stores energy rather than dissipating it.
Watch out Z is impedance, the total opposition including both R and X; Y is admittance (the reciprocal of impedance) and B is susceptance (the reciprocal of reactance), which show up in parallel circuit math.
R and X add up to Z. Flip them over and you get G, B and Y on the admittance side.