Some, like @denvercoder, have been in the industry for years and are yet to be one in name, others have had the title after a couple of years in the industry. It might be coveted or feared:

What does being a senior software developer mean?

TL;DR: It’s just a title!

The Longer Version

Why does it matter?

Being senior often comes with recognition, most often with a pay raise: after having worked for a while, having a title and the pay that goes with it is important!

What does it entitle?

Being senior will require the following two ingredients:

Leadership

Taking matters in one’s hands through to the end… to the best of one’s abilities. This means being able to design a solution, making that piece of software work, not being stuck on an issue (thanks Google!), knowing when to look for external help!

Mentorship

Knowing what you know and acknowledging what you do not and then imparting this knowledge to others. This goes with documentation, code reviews, advocacy and finding materials to stay on top of technologies and tools.

Last Words

My take is: after having done any kind of activity for a while, I have something to show for it!

Live your best life! ‘Being a senior’ is just a title: you accumulate knowledge and put it to work. This does not mean that your own definition of being a senior will match the expectations of your position; it means instead that your being a senior is valuable and that you need to find a place where it will fit!

References

What Is a Senior Developer and How Can I Become One? on freeCodeCamp.org

Same as what is written above but different (I know).

What exactly does it mean to be a “senior” software engineer (Stack Overflow Podcast)

Jocelyn shares her own journey

Recruiting, Interviewing, and Hiring Senior Developer Talent (InfoQ Live Roundtable)

Going further about the search for senior developers