\begin{definition}[Definition:Riemannian Submersion]
Let $\struct {\tilde M, \tilde g}$, $\struct {M, g}$ be Riemannian manifolds.
Let $\pi : \tilde M \to M$ be a smooth submersion.
Let $x \in \tilde M$, $p \in M$ be points in $\tilde M$ and $M$.
Let $H_x$ be a horizontal tangent space at $x$.
Let $T_p M$ be a tangent space of $M$ at $p \in M$.
Let $g_p$ be a Riemannian metric at $p$.
Suppose for every $x$ the differential $d \pi_x$ restricts to a linear isometry from $H_x$ to $T_{\map \pi x} M$:
:$\forall v, w \in H_x : \map {\tilde g_x} {v, w} = \map {g_{\map \pi x} } { \map {\d \pi_x} v, \map {\d \pi_x} w}$
Then $\pi$ is said to be a '''Riemannian submersion'''.
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\end{definition}