The notation 
denotes the algebraic closure of the rational numbers 
. This is equivalent to the set of algebraic numbers, sometimes denoted 
.
OverscriptBox[Q, _]
See also
Algebraic Number, Algebraics, QExplore with Wolfram|Alpha
References
Nesterenko, Yu. V. A Course on Algebraic Independence: Lectures at IHP 1999. Unpublished manuscript. 1999.Cite this as:
Weisstein, Eric W. "OverscriptBox[Q, _]." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Q-Bar.html