  List coloring generalizes graph coloring by requiring the color of a vertex
to be selected from a list of colors specific to that vertex. One refinement of
list coloring, called choosability with separation, requires that the
intersection of adjacent lists is sufficiently small. We introduce a new
refinement, called choosability with union separation, where we require that
the union of adjacent lists is sufficiently large. For $t \geq k$, a
$(k,t)$-list assignment is a list assignment $L$ where $|L(v)| \geq k$ for all
vertices $v$ and $|L(u)\cup L(v)| \geq t$ for all edges $uv$. A graph is
$(k,t)$-choosable if there is a proper coloring for every $(k,t)$-list
assignment. We explore this concept through examples of graphs that are not
$(k,t)$-choosable, demonstrating sparsity conditions that imply a graph is
$(k,t)$-choosable, and proving that all planar graphs are $(3,11)$-choosable
and $(4,9)$-choosable.
