Erg
An erg is the unit of energy in the centimetre-gram-second (CGS) system of units, symbol "erg". The erg is a quite small unit, equal to 1 × 10-7 jouless.
Erg is also a common name for an indoor rower, particularly those manufactured by the Concept2 company, taken from its purpose of measuring the amount of work done during training (as 'Erg' is short for 'Ergometer').
An erg is also a type of large sand dune.
The ergs were fictional "force field creatures" in Dan Simmons' Hyperion Cantos, used to drive Treeships through space.