styx

As proper nouns the difference between rubicon and styx

is that rubicon is an ancient Latin name for a small river in northern Italy which flows into the Adriatic Sea. It marked the boundary between the Roman province of Gaul and the Roman heartland. Its crossing by Julius Caesar in 49 BC began a civil war while Styx is the river, in Hades, over which the souls of the dead are ferried by Charon.

As a noun rubicon

is a limit that when exceeded, or an action that when taken, cannot be reversed.

As a noun taxonomy

is the science or the technique used to make a classification.

As a proper noun styx is

(greek mythology) the river, in hades, over which the souls of the dead are ferried by charon.