CATO stands for (or is alleged to stand for); Catastrophically Aborted Take-Off. A CATO is generally described as a motor failure, or what motor manfacturers more euphemistically call a casing failure or rupture due to overpressure. In layman's terms - the motor blows up.

CATOs are few and far between, but do occur, and for this reason, the safe distances recommended for each motor class make a lot of sense.