This is something I saw on Twitter. I find this is a really good question.

What makes a senior software engineer stand out from a software engineer ?

Soullessness, crushed hopes and dreams,

Salary

Software engineer: oh I have a cool new idea!

Senior software engineer: the idea is neither new because I've seen that three times, and it isn't cool either. First time, disaster X happened, second time disaster Y, and the third time, people became aware of the previous disasters mid-project and at least abandoned the idea before wasting even more money on it.

Software engineer: I think we should add this and this feature.

Senior software engineer: Makes sense, BUT you have to think about this and this case and how that will interact with features A, B and C. Otherwise, you'll need to redesign the whole thing mid-project.

Software engineer: I've been debugging that shit for two weeks and still can't find the problem!

Senior software engineer: Hmm, looks similar to something I had 15 years ago in a totally different tech stack. Let's see whether there's some equivalent here. (...) Oh, BINGO.

Its difficult to say, when is someone a senior? Im a senior according to my company, but Ive only two years work experience, and on of which was an internship+traineeship.

The ability to architect a solution to a problem well rather than solve a problem well. Ie create architecture that is good and expandable.

That and know more shit :)

Error count.