The term and the concept of observability aren't new at all. Observability first appeared in the world of engineering and control theory. Although the definition depends on whom you’re asking, some common patterns and understandings are emerging. Mostly it’s a new buzzword by the AIops providers and some thought leaders in the cloudops space (meaning me).
At its essence, observability is the measure of how well internal system states can be inferred from knowledge of all external data and states. It’s a bit different than monitoring which is something you do (a verb); observability is an attribute of a system (a noun).
If that’s a bit confusing, here's a more pragmatic definition: the ability to leverage the concept of observability and the tools that support that concept. Most monitoring and operations tools (such as AIops tools) claim some role in observability. However in some respects, to me it's the same old bourbon (monitoring) in a much fancier bottle (observability). I look at it with a bit of well-placed skepticism.