I am reading this paper, and the following definition of geodesic came up one line to the next without defining the symbol $\delta:$
By definition, a geodesic line is a line of minimal length, for which $\delta\int \mathrm ds=0$ for fixed extremities of the line.
I have some idea of what a geodesic is in terms of, for instance, a line that when parameterized by arc length it shows only accelerations along the normal vector to the surface, etc.
But I don't know what $\delta$ stands for in the equation above, and I'm not sure I would understand the formula if I knew. So can I get an explanation?