An 
set is a subset 
of the real
numbers that is the union of a countable
set of closed sets. The name 
comes from French: The F stands for fermé,
meaning "closed," while the 
stands for somme, meaning "sum."
This definition is dual to that of a Gdelta set in the sense that 
is an 
set iff
its complement 
is a Gdelta
set.
Both 
sets and Gdelta
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.
