Word To Use Today: boulder.

A boulder is a stone rounded by erosion and moved from its original position.

Something like this:

This is an glacial erratic boulder (which doesn't imply that it's unreliable, but that it's been moved by a glacier). Photo by Donald Perkins of a memorial to the Geologist Sir Andrew Crombie Ramsey at Eglwys Sadwrn Sant, Wales

It has to be more than 256 mm across or else it's a cobble.

The real reason for featuring this word on The Word Den, however, is its derivation.

So, without further ado:

Word To Use Today: boulder. This word comes from the Old Swedish bulder, which means rumbling. 

Cool, huh?