(mathematics, stochastic processes) A square matrix whose rows consist of nonnegative real numbers, with each row summing to 1. Used to describe the transitions of a Markov chain; its element in the i'th row and j'th column describes the probability of moving from state i to state j in one time step.

(physics) In the Standard Model, a unitary matrix that specifies the mismatch of quantum states of quarks when they propagate freely and when they take part in the weak interactions. It is important in the understanding of CP violation.