An ambient isotopy from an embedding of a manifold 
in 
to another is a homotopy of self
diffeomorphisms (or isomorphisms,
or piecewise-linear transformations, etc.) of 
, starting at the identity map,
such that the "last" diffeomorphism
compounded with the first embedding of 
is the second embedding of 
. In other words, an ambient isotopy is like an isotopy
except that instead of distorting the embedding, the whole ambient space
is being stretched and distorted and the embedding is just "coming along for
the ride." For smooth manifolds, a map
is isotopic iff it is ambiently
isotopic.
For knots, the equivalence of manifolds under continuous deformation is independent of the embedding space. Knots of opposite chirality have ambient isotopy, but not regular isotopy.
Knots." Math. Intell. 20, 33-48,
Fall 1998.