HP Designjet Printers - Definition of Delta E

What is the definition of Delta E as stated for Media and Ink Specifications for HP Designjet printers?

If there are two identical patches and one patches’ hue, saturation, or luminance value, is changed, Delta E is known as the point at which the standard observer in a very specific viewing environment can detect a change. If the Delta E value is multiplied by five, you will have a variation of five Delta E and so on. Again, this value is only useful for the specific viewing environment under which the test was done.

As a point of reference, most viewers find a Delta E of six or less to be an acceptable deviation in a commercial printing environment.