[abramN]

demonstrated expertise in one or more domains, can guide other professionals, sets and enforces standards, knows how to manage up and communicate to stakeholders, IT project management knowledge

[mcraenich]

This is a good answer although I think someone with this ability is approaching the level of lead. It's an ideal senior, but many seniors don't demonstrate all of these qualities.

At a bare minimum I think you're looking at someone who has a solid understanding of the code base and business they're working in.