What is an urn?

I can think of exactly two times when the word urn is used: as a container for someone’s ashes, and as a container for colored balls.  Since I’ve never physically seen an urn that has balls in it, it makes me wonder – when did that become such a standard in probability problems?  Why are the balls in an urn in the first place?

When I asked that very question, TwoPi mentioned “surmounted” as another example of an English word that seems to be used exclusively in one context:  to describe Norman Windows (a window in the shape of a rectangle surmounted by a semicircle).   “surmountable” is more common, and “insurmountable” even more so, so I suppose “surmounted” does actually appear in a related context (as in “that difficulty has been surmounted”), but it’s still relatively unusual.  I suspect that there are other words, English words as opposed to mathematical terms, that just don’t show up very often outside of the exercises in a text.

Here is a Norman Window, by the way, from Notre-Dame d’Étretat in Étretat, France.