Is it API surface simple ?
Is it data model simple ?
Is it Lines of Code short ?
Is it extensible with small core and huge plugins system ? ...
Is Wordpress simple ? Is MERN simple ? Is Rails simple ?
There's no answer here.
That's it. There's no "absolute simplicity", it depends on context.
Simplicity, like love, justice, elegance, etc are hard to define, but EASY to see.
How you know something is "complex"? When is not simple. When you know something is "complicated?", when is not simple, and not even complex.
The fact some concepts are hard to articulate is not a big problem most of time.
What is needed, is to ASK IT. Ask yourself "is this simple?" "no, we think is complicated". Pump! that's all.
You don't need to get lost on definitions to know. You only need to ask.
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P.D I don't argue against the importance of definitions... and sometimes people are sooooo lost or the project is sooooo messy that actually is necessary to make a mental check to make clear to everybody or ourselves, like @revskill say, to be more precise in what we are talking about. But most of time, is a problem of FAULTY COMMUNICATION not on words!
I've read everything I can about measuring complexity. I'm just not smart enough to understand most of it. So "simple" is still like pornography for me: I can't define it, but I know it when I see it.