Let (M,g)(M,g) be a with \nabla. A smooth curve γ:IM\gamma:I\to M is a geodesic if γ˙γ˙=0\nabla_{\dot\gamma}\dot\gamma=0 along II.

Existence and speed

Its speed γ˙(t)g\|\dot\gamma(t)\|_g is constant. Given (x,v)TM(x,v)\in TM, there is a unique geodesic with γ(0)=x\gamma(0)=x and γ˙(0)=v\dot\gamma(0)=v on its maximal interval of existence. If vg=1\|v\|_g=1, it is unit speed.

Reference