Normally hyperbolic invariant manifold

A normally hyperbolic invariant manifold (NHIM) is a natural generalization of a hyperbolic fixed point and a hyperbolic set. The difference can be described heuristically as follows: For a manifold Λ {\displaystyle \Lambda } to be normally hyperbolic we are allowed to assume that the dynamics of Λ {\displaystyle \Lambda } itself is neutral compared with the dynamics nearby, which is not allowed for a hyperbolic set.

Source: Wikipedia — Normally hyperbolic invariant manifold (CC BY-SA 4.0)