An erg is a unit of measurement or energy or work that is the same as 10^-7 joules. It formed part of the centimetre-gram-second system of units (CGS system of measurement which is no longer used).

An erg is about the amount of work done (or energy consumed) by one common housefly performing a "press-up", the leg-bending dip that brings its mouth to the surface on which it stands and back up. ^[1]

1. ↑ Filippenko, Alex, Understanding the Universe (of The Great Courses, on DVD), Lecture 44, time 24:30, The Teaching Company, Chantilly, VA, USA, 2007