2-sided

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In topology, a compact codimension one submanifold F of a manifold M is said to be 2-sided in M when there is an embedding

h\colon F\times [-1,1]\to M

with h(x,0) = x for each x\in F and

h(F\times [-1,1])\cap \partial M=h(\partial F\times [-1,1]).

This means, for example that a curve in a surface is 2-sided if it has a tubular neighborhood which is a cartesian product of the curve times an interval.

A curve which is not 2-sided is called 1-sided.