What does the expression #x in RR# mean ?
3 Answers
A real number is any rational or irrational number.
For example:
It means that
It usually means:
"
#x# is a member of the set of real numbers"
or more simply:
"
#x# is a real number"
Explanation:

#RR# usually denotes the set of Real numbers. 
#in# denotes membership.
So
Related expressions are:

#AA x in RR" "# meaning "for all#x# in the set of real numbers". in other words: "for all real numbers#x# ". 
#EE x in RR : ..." "# meaning "there exists a member#x# in the set of real numbers such that ..." or "there exists a real number#x# such that ...".
In some kinds of constructive mathematics, where speaking of "the set of Real numbers" is considered a little presumptuous, the expression "