A relation between compact boundaryless manifolds (also called closed manifolds). Two closed manifolds
are bordant iff their disjoint union is the boundary of a
compact 
-manifold.
Roughly, two manifolds are bordant if together they
form the boundary of a manifold. The word bordism is
now used in place of the original term cobordism.
Bordism
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References
Budney,
R. "The Bordism Project." http://www.math.cornell.edu/~rybu/bordism/bordism.htmlReferenced on Wolfram|Alpha
BordismCite this as:
Weisstein, Eric W. "Bordism." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Bordism.html