If I were to be cynical, I'd say age and wage (and the feeling of knowing it all).
However, contrary to some of my previous experiences, a senior is that person that has an answer to most of your questions (and the disposition to answer them). It's that person in the office that can pull a project or a team on his own on the long term, without major fuck-ups and with a clever solution for all unexpected problems.
And if you're interested in the more superficial description: human resources would call a senior someone who's been mastering his domain for at least 3 years.
However, contrary to some of my previous experiences, a senior is that person that has an answer to most of your questions (and the disposition to answer them). It's that person in the office that can pull a project or a team on his own on the long term, without major fuck-ups and with a clever solution for all unexpected problems.
And if you're interested in the more superficial description: human resources would call a senior someone who's been mastering his domain for at least 3 years.