G-delta set

In the mathematical field of topology, a G_δ set is a subset of a topological space that is a countable intersection of open sets. The notation originated in Germany with G for (German: area, or neighbourhood) meaning open set in this case and δ for (German: intersection). The term inner limiting set is also used. G_δ sets, and their dual F_σ sets, are the second level of the Borel hierarchy.

In a topological space a G_δ set is a countable intersection of open sets. The G_δ sets are exactly the level ${\displaystyle \mathbf {\Pi } _{2}^{0}}$ sets of the Borel hierarchy.

A more elaborate example of a G_δ set is given by the following theorem:

Theorem: The set ${\displaystyle D=\left\{f\in C([0,1]):f{\text{ is not differentiable at any point of }}[0,1]\right\}}$ contains a dense G_δ subset of the metric space ${\displaystyle C([0,1])}$. (See Weierstrass function#Density of nowhere-differentiable functions.)

...
Wikipedia