Is it plagiarism when copying something that you have written before?

Plagiarism and self-plagiarism sound similar, but are importantly distinct things.

Plagiarism is if you use other people's work as your own. You deprive them of credit and claim credit for yourself which you do not deserve.

Self-Plagiarism does not deprive the author (you) from credit, but the offence here is that the the "present-you" claims credit that the "past-you" already collected.

This is an assessment offence because you would get a double mark for a single piece of work. You could, of course, quote yourself (properly cited!), in which case, you would be perfectly fine, but you would only gain marks for new work done, which, in case of a full copy, would still be zero. However, in a sensible school policy it should not be punishable for plagiarism in the conventional sense, since it was you in the first place who wrote the text.

As a student, I have reused software libraries I wrote in other coursework, properly cited, of course. It would be a waste of time to redo mechanical work, and permitted me to get much further. With an essay, nothing stops you, of course, to build upon existing writing (properly cited) to develop your argument further.

Yes, it is called self-plagiarism. This happens when you reuse significant portions of an earlier work without acknowledging this. This can be a major concern if the prior work is copyrighted as it infringes on the rights of the publisher.

If the work comes from any other sources, it is wisest to cite where it came from, even if it came from you.

Terms used are self-plagiarism (see @Darrin Thomas) or autoplagiarism. One of its issues in scientific publishing is "duplicate" or "multiple publications", that brings very little novelty to the subject. I have witnessed the same paper published in 3 or 4 different conferences.

Such behavior are now ruled by scientific societies. For instance at IEEE (note that I am quoting, the original source is under the link):

IEEE Publications has long maintained the policy that verbatim copying of another's work (plagiarism) is unacceptable author conduct. In November 2002, the IEEE Board of Directors approved a new policy on Duplicate Publication and Self-Plagiarism. [...] authors should only submit original work that has neither appeared elsewhere for publication, nor which is under review for another refereed publication. If authors have used their own previously published work(s) as a basis for a new submission, they are required to cite the previous work(s) and very briefly indicate how the new submission offers substantial novel contributions beyond those of the previously published work(s).

One legal reason could be that such societies own some rights regarding the copyright. Not a proof, but in the Wikipedia section on Self-plagiarism, you can find:

In addition there can be a copyright issue if copyright of the prior work has been transferred to another entity

If your work is not published or evaluated, reuse of sentences you have written sounds like a fair use. If submitting an essay induces a legal framework (grade, assignement), you should take some care. You "own exact sentences" are a matter of quantity. At least, it is important that your fairness can not to discussed. Thus, if you mention that you already have worked on a similar topic (with references), you have less chances to be blamed for hiding important information.

As a side note, apart from the time gain, I strongly suggest you to rewrite, without looking too much at the original source, apart to make sure you have not forgotten ideas. Your though has evolved by thinking about it once, and writing it, and reformulating is a good way to improve your line of reasoning and the clarity of your written expression.