I don't think the above definition
of 'offside' is quite correct. There are various exception
s, as follows:
Put all that into a single sentence and you definitely wouldn't win any awards from the Plain English Campaign.
It has been pointed out to me that FIFA have the official version of the rule here: http://www.fifa2.com/scripts/runisa.dll?s7.131714:GP::67173+refs/laws/law11