Is it plagiarism if I use a text I wrote myself but put on the internet before?
I think this is an interesting question that depends on separating three concepts that sometimes get intertwined:


plagiarism
copyright
legality


Plagiarism refers to using something without properly indicating its source.

Copyright refers to having the right to use material elsewhere.

Legality refers to whether you're violating a law or committing a crime by doing something.

If you've posted something anywhere (and especially to an openly editable place like wikipedia or SE), then there's possible issues with reusing it on each of the fronts.

With respect to plagiarism, what matters is if the prior posting counts as "published" or "submitted" for the purposes of the item in question. (The former being the standard for journals and books; the latter being the standard for classes -- though I suspect most classes would care if you submitted something you already had published).

For copyright, you may not have the legal right to use something in its entirety even if you're the one who wrote it. This would depend on which copy rights your retained from your first publication. If it's SE, for instance, you're under Creative Commons. Wikipedia seems to have their own thing (I'm not a lawyer and can't entirely parse their policy).

Finally, none of this means that you're doing anything criminal, but if you were publishing for money, you could have liability if you violate the copyright.

Two confusing issues are that copyrights can speak as if they create citation obligations (they don't) and act as if their violation is a crime, but at least in the US its only a crime when done for profit.