A 
set in 
is a subset 
of the real numbers
that is the intersection of a countable
set of open sets. The name 
comes from German: The G stands for Gebiet, meaning
"area," while the 
stands for Durchschnitt, meaning "intersection."
This definition is dual to that of an Fsigma set in the sense that 
is a 
set iff
its complement 
is an Fsigma
set.
Both 
sets and Fsigma
sets are fundamental to measure theory. In
particular, both types of sets are Borel
sets and belong to the second level of the Borel
hierarchy.
