Definition of "erratic"

The term for a displaced boulder is an erratic, but in the nineteenth century the expression seemed to apply more often to the theories than to the rocks.

2003, Bill Bryson, A Short History of Nearly Everything, BCA, page 372

During the last ice-age, massive stones were carried for miles by the scouring glaciers, only to be left, like passengers at the end of the line, when the glaciers retreated. Stranded in their new surroundings with rocks with which they share no common geology, their out-of-place-ness is evoked by their name: “erratics”.

2015 May 4, Dominick Tyler, “10 UK landscape features that you’ve probably never heard of”, in The Guardian‎