Is rebreather and debreather the same thing?
No. A rebreather is used in SCUBA diving. It takes advantage of the fact that there is still a significant level of oxygen remaining in expired air. This oxygen is wasted in a conventional SCUBA system where all exhaled air is allowed to escape and bubble to the surface. In a rebreather system the carbon dioxide from exhalations is scrubbed of carbon dioxide, and a small amount of oxygen is fed into the system to keep the proportion of gases (primarily nitrogen and oxygen) at a safe breathable level. In essence, you are just breathing the same air in a closed system, with CO
2 removed and O
2 added.
The debreather operates on the same principle, except that there is no oxygen top up. CO
2 is still scrubbed to prevent the hypercacnic reaction kicking in, and so the proportion of nitrogen in the breathed air just keeps climbing until death occurs.