Definition:  two modes for which the total power crossing any transverse crosssection is equal to the total flux, through that crosssection, of the sum of the individual Poynting vectors of the modes; in other terms, the total flux across that crosssection, of the sum of the two crossproducts ${\overrightarrow{E}}_{1}\times {\overrightarrow{H}}_{2}+{\overrightarrow{E}}_{2}\times {\overrightarrow{H}}_{1}$ in the total Poynting vector, is zero
