This is very much the perspective I have taken over the years. When I'm hiring a senior developer, I'm looking for someone who will step in and own the project they're given. If I had to pick a single expression, especially one recognizable to this crowd, I would say it is someone who can execute. I also tend to view senior developers almost as internal consultants (good ones) because they should be willing and able to step away from they keyboard and talk to a stakeholder to discern what the business really needs without needing someone else to act as an intermediary.

Where I do disagree just a little bit with mrmekon, is that I tend to look for people who have experience with the necessary stack. I would be hesitant to hire a senior developer for a C#/MVC position if they have (broadly) never used C# or never done web development, for example. Modern stacks have a lot of moving parts, and I would prefer senior developers to understand how to troubleshoot, tune, upgrade, test, and deploy on the needed stack. These are things that a very strong developer could remediate within a few projects, though, which is why I only slightly disagree with mrmekon.