In reply to zimpara:
It's hard to say exactly where you stole that quote from, since it is repeated all over the web...
Softshell is a vague and poorly-defined concept, and I'm not sure the idea of a 'softshell fabric' always makes great sense. You could make a softshell jacket from fleece or fleece-like material, such as Polartec 200, Polartec High Loft, Polartec Thermal Pro or even Polartec Alpha, and either a membrane or a face fabric such as Pertex Microlight, without ever using a 'softshell' fabric. Or you could use a non-membrane wind-blocking fleece material (one with a very dense arrangement of fibres, like Polartec Wind Pro, which might make a softshell jacket all on its own (although you might still add an face fabric) - is this a 'softshell' material? Wind Pro is definitely still a fleece.
What about Polartec Windbloc, Polartec Powershield or Polartec Powershield Pro (which is almost a waterproof material more akin to Neoshell) which incorporate membranes to make them wind/water resistant?
'A class of knit of woven materials, made from polyester and nylon' is a terrible definition for a number of reasons (albeit not your definition).
For a start not all are made from a mixture of polyester and nylon. By your own definition, Powerstretch is not a softshell since it consists of a mixture of polyester and Spandex (apparently a 'polyester-polyurethane co-polymer'). Powerstretch Pro adds a nylon layer on the outside to protect against abrasion or something like that - does that make it a softshell?
Secondly it doesn't actually give you any information about what makes a 'knit of woven materials' a softshell or not. I can tell you that fish are a class of animal, but I haven't told you anything about how to work out whether something is a fish or not (other than it must be an animal)...
Softshell and fleece are not mutually exclusive and are different classes of concept. Dubious analogy: you make a hardshell from a waterproof fabric; you make a softshell from a wind/water resistant fabric (which could include/be a fleece). Other people may have a different idea of what softshell is
e.g.
http://andy-kirkpatrick.com/articles/view/the_best_softshell_in_the_world
http://andy-kirkpatrick.com/articles/view/cut_the_crap1
To be clear - the reason you were talking rubbish was your lack of understanding of what Polartec is.
And yes, I am procrastinating