The queue number 
of a graph 
is the minimum number of queues in a queue layout of 
. A queue layout consists of a total
order of the graph vertices together with a set partition of the graph
edges into queues such that no two edges in the same queue are nested. Here,
two edges 
and 
that share no endpoint and have 
and 
in the vertex ordering are nested if 
or 
(Heath and Rosenberg 1992, Dujmović and
Wood 2004).
Queue number is analogous to book thickness, also known as stacknumber, with the non-nesting condition for queues replacing the non-crossing condition for stacks.
