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 ≥ k , a ( k , t ) -list assignment is a list assignment L where | L ( v ) | ≥ k for all vertices v and | L ( u ) ∪ L ( v ) | ≥ t for all edges u v . 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. [ABSTRACT FROM AUTHOR]