Ignition is the process by which a fire starts, marking the point at which a material or substance reaches its critical temperature and begins to combust. It is the initial phase of the fire cycle, where heat, applied through sources such as natural or human ignition provides enough energy to overcome the material's resistance to burning. The role of ignition in fires is crucial because it triggers the chain reaction that allows a fire to sustain itself and spread. Once ignition occurs, the fire gains momentum as it continues to consume surrounding fuel, fueled by the presence of oxygen, and can escalate into a wildfire if environmental conditions allow.