I would have tended to agree with Microburst, whose name does suggest he ought to know something about this. ;-)
However, looking up the "Airplane Upset Recovery Training Aid Revision 2" it, at least, defines windshear as "Wind variations at low altitude". Interestingly it doesn't define "gust" at all. It then contradicts itself by later discussing "Windshear at the boundaries of the jet-stream" which clearly isn't a low altitude phenomenon!
AC120-50 specified "low altitude windshear" which allows for the possibility of it occurring elsewhere. AC00-54 is a bit more useful in defining windshear as "any rapid change in wind direction or velocity". There's an unspoken "sustained" in there somewhere...