As an additional historical note: in Unix, load averages were introduced in 3BSD, and at that time they included processes in disk IO wait and other theoretically short-term waits that weren't interruptible. This definition was carried through the BSD series and onward into Unixes derived from them, such as the initial versions of SunOS and Ultrix. At some point (perhaps SunOS 3 to SunOS 4, perhaps later), the SunOS/Solaris definition changed to be purely runable processes.
(I'm not sure what System V derived Unixes such as Irix, HP-UX, and so on did, and their kernel source is not readily available online for spelunking.)
As of early 2016 when I last looked at this, the situation on FreeBSD, OpenBSD, and NetBSD was somewhat tangled. FreeBSD load average only included runable processes, but NetBSD and OpenBSD counted some sleeping or waiting processes as well.