To me a senior engineer is one who can lead a team in the design, documentation, implementation, deployment and maintenance of a system which realizes a quantifiable ROI for the organization and they've preferably done this with different tech stacks.

These can be internal systems which focus on more efficient processes for the organization or external facing systems which generate revenue.

I've worked with a LOT of engineers that have never built and delivered anything of real value. That's not to blame them, as there's a tremendous amount of waste in both the public and private sector driven by middle management layers.

But in my book, if one wants the title Senior Engineer then one has to have built and shipped something that generates more revenue (directly or indirectly) than what the system cost to build and maintain.