In an existing system some combination of these attributes:

- High quality (e.g. low number of issues hit by customers, resilient to failures, efficient, secure etc.)

- Easy to maintain (well organized, broken down in a sensible way into components or layers)

- Easy to extend/adapt to future requirements (i.e. the designer was able to anticipate the likely direction of the system and account for that in the design)

Automated testing feels a bit orthogonal to me but a system that is easy to test is likely one with a better architecture. It's not strictly part of what I'd call architecture.

Less different technologies - YES!

Runs on fewer machines is a sign of an efficient/performant design. Less well designed systems exhibit bloat that is often made up for by running on more machines.