My definitions are basically: Junior developers need supervision because left to their own devices they'll screw things up horribly; normal developers can produce good code independently; and Senior developers are able to catch the mistakes the Junior developers are making and set them on the right path.
I see; that's L3, L4, and L5 progression in a nutshell at Google - although leaving L3s alone doesn't _guarantee_ something will go wrong, it was more that there was no way for them to figure out optimal solutions without help thanks to the sheer complexity of Google.
I'd say the same held true at Amazon but I was in groups which were, at the time, at the periphery of the company's engineering efforts - we didn't have any associated principals to talk to, and maybe one SDE3/L6 to 10 SDE2/L5s mixed with SDE1/L4s.
I would say* under these definitions L3 is junior; what the industry calls senior is somewhere between L4 and L5. L5s at Google are expected to mentor L3s and L4s but also to design systems, break down into tasks, and coordinate tasks between teams and engineers.
If you were a senior engineer at a 50-person startup you would commonly get hired at L4.
* I left Google 18 months ago; also, Google is a large company, and while they strive for uniformity across teams, the levels aren't really quite the same company-wide.
