What is an eigenstate and how does it relate to quantum mechanics?

An eigenstate in quantum mechanics is a state in which a physical system is in a definite, well-defined state with respect to a particular observable quantity, such as position or momentum. In simpler terms, it is a state in which a property of a particle is known with certainty. Eigenstates play a crucial role in quantum mechanics as they provide a way to describe and understand the behavior of particles at the quantum level.