Check out: VMware’s multi-cloud toolkit for Kubernetes.
And here’s my answer:
“Multi-cloud” mostly means, within one organization, using different types of infrastructure. Mostly simply, public and private cloud. You could say “traditional datacenter” (mainframes, AS/400s, virtualized x86…I guess?). And, you know “edge” (which is sort of an evolving concept, but real enough as a category).
If we narrow down to software, though, I think of “multi-cloud” a little different. Here, think about multi-cloud as meaning you can run your apps on whatever infrastructure you need to because you have a common runtime layer and, probably, management tools. This is sort of like Java’s “write once, run anywhere” vision. You can also throw in using services from different clouds and your own 0n-premises stuff.