QED is a predecessor to ed and vi, one of the first to use regular expressions.
It lies somewhat in between vi and Emacs in that its commands more resemble those of vi (which is essentially a successor), but where, like Emacs, it supports having multiple buffers in memory. Going beyond both, in terms of "computing power", it offers the capability to have commands that operate on sets of buffers (hence operating on multiple files) all at once.
Historically, it should probably be particularly contrasted with TECO .
They are roughly contemporary
The first version of Emacs was created as a set of TECO macros, so there is a whole set of similar editors down that path.
In contrast, QED had, as successors, Unix-related editors such as ed, sed, vi, sam, acme.
There are likely, today, vanishingly tiny sets (possibly null) of programmers still using TECO and QED, but both editors have been hugely influential (despite being fairly ill-known in modern times).
An incomplete history of the QED Text Editor by Dennis Ritchie
QED Archive - archive of source code of various versions of QED.
Collected as a result of discussions on TUHS
CTS QED source code
This was derived from USENIX tape 80.1, and patched (a bit) to build on modern Linux. This is probably the easiest QED to get running.
This Plan 9-sourced text editor has a similar operating model, and is arguably a successor. (I believe that Kernighan and Pike both consider it to be the successor that they use, which ought to be a pretty decent argument in this regard...