I don't know about other companies, but here's a generalised description of officially what a senior software engineer does.

- Relevant tertiary qualification - min 5 years experience - long list of proficiency relevant to particular area of work - Proven ability to take a lead role in the design and development of a feature from start to completion - Establish technical feasibility of assigned projects - Uses diagramming appropriately to convey technical details and features - Create, Document, Maintain technical design approaches - Mentor other developers - Lead debugging investigations and report results - Present results of research and investigation, to team meetings, or at conferences, etc.

+ Everything a normal/jr dev does

I think it's a pretty good/comprehensive definition, and it seems clear enough to me in my work environment.