I would say being a senior developer is a combination of the following:

• has experience
• knows how to do many tasks without looking up how to do it
• if they don't remember how to do it, they are able to quickly find the information
• when working on a completely new type of task, they aren't afraid to jump in and try it before asking for assistance. Thinking of myself as a junior developer, I was much more apprehensive about starting new types of tasks, and needed more hand-holding. This has gone away a bit with experience.
• is able to assist junior developers in their learning
• is able to communicate well with the team
• is able to look at the bigger picture of the task, and see what other areas it might affect