 Cobordism

A cobordism from a closed oriented $n$-manifold $X_0$ to $X_1$ is a compact oriented $(n+1)$-manifold $W$ whose oriented boundary is $-X_0\bigsqcup X_1$.

Cobordism is a concept from the field of topology, particularly in algebraic topology, that studies the relationships between manifolds. In simple terms, cobordism provides a way to classify manifolds based on their boundaries and their relationships to each other.