What immediately comes to mind when you hear the term, "hybrid-cloud visibility?"

This is the current buzzword for salesmen when talking to potential clients.  I see it as a bridge between on-prem and cloud environments working within an ecosystem that integrates physical to virtual with visibility into your infrastructure, systems, applications, traffic, logging, etc.. This has been in place for years now, it's just a "new buzzword" to get start the conversation with.

Asset/Inventory management software that can discover endpoints in the cloud, where endpoints can be VMs, database instances, webservices, storage accounts etc

Buzzword.
But seriously what Mark said below 👇🏻

The ability to view any number of performance and availability criteria across multiple cloud instances (I.e., VMware and AWS).  As performance on a data base in one cloud could impact application performance served from a 2nd cloud, a holistic view of relationships and click to I/O performance is critical. 
Also applicable:
1. Security 
2. Patching
3. Unused instances
4. etc.

Access and control of all network traffic across my distributed environment? I think this speaks to reducing potential blind spots.