Quantum criticality is a state or phenomenon in condensed matter physics occurring at a quantum critical point, which represents a phase transition driven by quantum fluctuations at zero temperature. As a research concept, it investigates the singular behavior and emergent properties of materials near such points, characterized by diverging correlation lengths and timescales that can manifest at finite temperatures, often leading to non-Fermi liquid behavior and novel ground states. Its significance lies in providing a framework for understanding strongly correlated electron systems and emergent phenomena like unconventional superconductivity.