  The class of local invertible operations is defined and the invariance of
entanglement under such operations is established. For the quantification of
entanglement, universal entanglement measures are defined, which are invariant
under local invertible transformations. They quantify entanglement in a very
general sense. It is shown that the Schmidt number is a universal entanglement
measure, which is most important for the general amount of entanglement. For
special applications, pseudo-measures are defined to quantify the useful
entanglement for a certain quantum task. The entanglement quantification is
further specified by operational measures, which include the accessible
observables by a given experimental setup.
