X11 uses the term "display" to refer to a one or more "screens" that
are connected to a set of input devices.  So if you had a Microvax II
with dual QDSS, each with a keyboard and mouse, you'd have :0 and :1
and two displays that could support two login sessions.  I'd call this
multi-headed.  If you have two screens being used by one person with
one set of input devices, you'd have :0.0 and :0.1, and I wouldn't
call it multiheaded.   I'd suggest just using "display" and "screen"
which are defined precisely within X11.

-- 
        Greg Troxel <gdt@ir.bbn.com>