Morse function

Etymology
Named after.

Noun

 * 1) A real-valued function on a differentiable manifold such that, at each point in its domain where the function's differential is zero, the function's Hessian is nonsingular.

Usage notes

 * Some sources require also for each $$t\in\mathbb{R}$$ that the preimage of $$\left(-\infty,t\right)$$ be compact, or that the preimage of $$t$$ contain at most one point with zero differential.