This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
The following definition has been posted to r.m.r. by Jack Hagerty,
(email@example.com) editor of the excellent r.m.r Glossary. For even more
complete information on the term CATO, refer to the glossary.
A motor failure, generally explosive, where all the propellant
is burned in a much shorter time than planned. This can be
a nozzle blow-out (loud, but basically harmless), an end-cap
blow-out (where all of the pyrotechnic force blows FORWARD
which usually does a pretty good job of removing any internal
structure including the recovery system) or a casing rupture
which has unpredictable, but usually devastating, effects.
Another form of CATO is an ejection failure caused by either
the delay train failing to burn or the ejection charge not
firing, but the result is the same: the model prangs.
Opinions on the meaning of the acronym range widely. Some
say it's not an acronym at all, but simply a contraction of
'catastrophic' and should be pronounced 'Cat-o' (which sounds
better than 'cata' over PA systems :-). Others maintain that
it is an acronym but disagree on the meaning, offering a
broad spectrum of 'CAtastrophic Take Off,' 'Catastrophically
Aborted Take Off,' 'Catastrophe At Take Off' and the self
referential 'CATO At Take Off.' The acronym crowd pronounces
it 'Kay-Tow', like the Green Hornet's side kick. It has been
pointed out, though, that all of the above are 'post-hoc'
definitions since LCO's were using the term over range PA
systems long before any formal acronym was established.