Bordism is 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." https://web.archive.org/web/19970613092454/http://math.cornell.edu:80/~rybu/bordism/bordism.html.Referenced on Wolfram|Alpha
BordismCite this as:
Weisstein, Eric W. "Bordism." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Bordism.html