In my opinion, a Senior dev is a reliable Developer that needs no supervision. They have a good understanding of their domain and their technologies. Think of them as a heat-seeking missile. You just point it at or near the target, fire it off and it will stear itself the rest of the way with high success. Typically a senior developer will have completed an entire SDLC of at least one product or at least been apart of the launch. Will have an nack for trouble shooting and enough experience to sound competent when people from outside the team ask questions.

Senior Developers are difficult to find because years of experience does not always correlate to ability in our field. My first year as a SWE I made less than half some of the “senior” devs made and within six months I was out performing many of them. I eventually got a really nice raise shortly after that time, but what I still would expect to be less then said “senior” devs. It's the nature of the beast I suppose.

In my experience, a senior developer needs to have at least 2 years of experience to meet all the above criteria. Most companies will have varying definitions and many fall into the trap of x years of experience where x is at least 7. They typically have a bunch of the “senior” Developers I mentioned earlier who are okay at best, but dilute the meaning of a senior dev.