This triggered me. Senior is a title that, like everything else in our Agile world, has been evolving to subsume activities that were once considered to be more specialized. Similar to how "devops" has combined sys admins and network engineers with development, and agile itself has combined "business analyst" with development, Senior now includes "team lead" and "architect". It's a pitiful state of affairs imo.
And we've done it to ourselves. Every time we as a collective have complained that X is a waste of time, this persons job has no value, we don't need them, guess what? The people in charge listened. I personally don't like that this has happened for the primary reason it makes my job harder.
"Junior" means you implement designs prepared by someone else. "Senior" means you are the one making the technical designs, to be implemented either by yourself or by someone in your team.
In these definitions, "design" refers to the overall structure of a program or system. The level where decisions like "do we hit the DB for every query or put a cache in between" are made.
These separations are completely arbitrary in practice. I've seen many seniors not doing any technical designs, and I've seen many juniors expected to stand on their own two feet when it comes to making technical designs, sometimes to the point of requiring those skills before getting a job.
Reality is, the title means hardly anything anymore except "this person has this many YoE" and whatever value the company attaches to the title in a given context. That's what has made the title so watered down.
Additionally, many graduates can make technical designs. They won't be the best. They won't think of everything. They likely don't have the foresight to think of problems happening along the way. But they are being taught how to convert requirements into a working solution during school years. I have a hard time believing "seniority" is defined by a few years of practical experience in something that was taught in school.
They are usually meaningful within a given company and become increasingly important as the company grows in size. Comparing titles between companies is usually pointless.
WTF does "being a senior" even mean?