It's easier to say what does not define a senior engineer. Here's a couple that come to mind:
age
work experience in years
amount of certificates collected
university grades
salary (though often these are well aligned)
tech skills
Wait, what? Looking at tech skills can't prove whether someone is fit for a senior role? Correct. When thinking of senior engineers I often reference John Allspaw:
"Being able to write a Bloom Filter in Erlang, or write multi-threaded C in your sleep is insufficient. None of that matters if no one wants to work with you. Mature engineers know that no matter how complete, elegant, or superior their designs are, it won’t matter if no one wants to work alongside them because they are assholes. Condescension, belittling, narcissism, and ego-boosting behavior send the message to other engineers (maybe tacitly) to stay away."
Let us become mature engineers instead of seniors.
It's easier to say what does not define a senior engineer. Here's a couple that come to mind:
Wait, what? Looking at tech skills can't prove whether someone is fit for a senior role? Correct. When thinking of senior engineers I often reference John Allspaw:
Let us become mature engineers instead of seniors.
I really liked your closing line:
This pretty much sums it.