One example would be in the List docs:
:unlet list "idem
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It's a latin term meaning "the same". In the quoted context, it just means the listed command does the same thing as the previously listed command, i.e.
remove item 3:
:let i = remove(list, 3) " remove item 3 :unlet list " idem
... :let i = remove(list, 3) " remove item 3 :unlet list " idem :let l = remove(list, 3, -1) " remove items 3 to last item :unlet list[3 : ] " idem ...
idem is to say that that line of code does the same as the line above.
This could be useful: https://en.wiktionary.org/wiki/idem