We introduce the notion of $n$-dimensional topological quantum field theory (TQFT) with defects as a symmetric monoidal functor on decorated stratified bordisms of dimension $n$. The familiar closed or open-closed TQFTs are special cases of defect TQFTs, and for $n=2$ and $n=3$ our general definition recovers what had previously been studied in the literature. Our main construction is that of generalised orbifolds for any $n$-dimensional defect TQFT: Given a defect TQFT $mathcal{Z}$, one obtains a new TQFT $mathcal{Z}_{mathcal{A}}$ by decorating the Poincare duals of triangulated bordisms with certain algebraic data $mathcal{A}$ and then evaluating with $mathcal{Z}$. The orbifold datum $mathcal{A}$ is constrained by demanding invariance under $n$-dimensional Pachner moves. This procedure generalises both state sum models and gauging of finite symmetry groups, for any $n$. After developing the general theory, we focus on the case $n=3$.
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