The erg is a CGS unit of energy equivalent to
the work done by the force of a dyne (a unit
of force) pushing over a centimeter distance.
An erg is one gram centimeter-squared per second-squared
(g·cm²/s²),
a dyne being the force necessary to accelerate a gram
one centimeter per second-squared.
107 ergs make a joule (the SI unit of energy).
Erg remains widely used in astrophysics, as is the CGS
system generally.