A Continuous-Time Markov Chain (CTMC) is a mathematical model used to describe systems that transition between different states over continuous time. In a CTMC, the future state of the system depends only on its current state, not on how it arrived there. This property is known as the Markov property, which simplifies the analysis of complex systems. CTMCs are widely used in various fields, including queueing theory, reliability engineering, and population dynamics. They are characterized by rates of transition between states, which can be represented using a transition rate matrix. This allows for the calculation of probabilities and expected times for state changes over time.