From my experience, someone can be said to be a "senior" engineer when he/she is proficient enough to be inserted into a team, brought up to speed via an onboar

From my experience, someone can be said to be a "senior" engineer when he/she is proficient enough to be inserted into a team, brought up to speed via an onboarding process and then is almost immediately productive, in a more-or-less independent fashion, ideally bringing new know-how to the team. But then again, it's not all about tech.