A perfectly reasonable-sounding explanation for an inexplicable situation; particularly when the factual explanation is undesirable.
Example:
Why some
work hasn't been done.
Real reason: because
you couldn't be bothered, or found something more
interesting (or
less difficult) to
do instead.
Excuse: because you were
abducted by
aliens who
gave you an
anal probe.
(Note: theexample should not generally be used, as it's, frankly, rubbish.)