Isoazimuth
The isoazimuth is the locus of the points on the Earth's surface whose initial orthodromic course with respect to a fixed point is constant.
That is, if the initial orthodromic course θ from the starting point S to the fixed point X is 80 degrees, the associated isoazimuth is formed by all points whose initial orthodromic course with respect to point X is 80°. The isoazimuth is written using the notation (X, θ).