The index is the distance away from the first element. So index 0 is the first element. Index 3 is 3 away from the first element, so the fourth element.
It's the difference between ordinal and cardinal numbers. In other words, counting and indexing.
Formally, ordinals start at zero too :-) [0]. We owe their latest definition to Von Neumann, but AFAIK, the former definitions were similar.
[...snip...]
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[0] This is meta-meta-contrarianism.
- The layman counts from 1
- The uptight programmer counts from zero, because Dijkstra said so (or so he thinks).
- The meta-contrarian (I used to be one) says fuck it, ordinals start at one.
- The meta-meta-contrarian reads Wikipedia[1], realizes he was formally wrong, and goes one step further in pedanticity, back to zero [0].
That being said, my brain prefers 1-indexing programming languages like Lua, Julia, R and Matlab...