# Definition A topological insulator is a type of [[quantum material]]. It is characterized by a [[Topological Invariant]], [[Bulk-Boundary Correspondence]] and [[Spin-Momentum Locking]]. Examples include [[Time-Reversal Symmetric Topological Insulator|time-reversal symmetric topological insulators]], [[antiferromagnetic topological insulator|antiferromagnetic topological insulators]] and [[Topological Crystalline Insulator|topological crystalline insulators]].