according to this this and owner seem to have the same meaning. Reading further I find that owner can refer to the enclosing class or closure. Is this the only difference? If so, why reserve an entire word just for that?
this refers to the enclosing class instance. owner refers to the directly enclosing object, which may or may not be the enclosing class instance. For example:
def x = { def y = { println this; println owner }; y() }
x()
Here this refers to the instance of the script class, and owner refers to x.