Bordism is a relation between compact boundaryless manifolds (also called closed
manifolds). Two closed manifolds of dimension 
are bordant iff
their disjoint union is the boundary of a compact
manifold of dimension 
. 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
See also
Complex CobordismExplore with Wolfram|Alpha
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