Plagiarism is the act of passing off work done by someone else as if it were your own. It even extends to your own work so reusing one assignment in another context will be counted as self-plagiarism. It does not have to be deliberate to come within this definition.
It can take a number of forms including:
How to avoid plagiarism:
You can read more about plagiarism.
You may be surprised to learn that it is possible to plagiarise yourself.
Self-plagiarism is when people reuse work that they have done before without citing it. So if you use reuse your own words or ideas in a subsequent piece of work without acknowledging it, you are plagiarising. This means that you must cite your own previous work in the same way that you would cite work by any other author.
There is more advice on this blog from Turnitin: Is recycling your own work plagiarism?