This isn’t plagiarism, because I have attributed it to the dictionary (Concise Oxford, to be precise). Had I simply typed that and presented it as my own thought, it would have been plagiarism.

Copying and pasting from a website is plagiarizing. Even the cosmetic bits about changing and deleting and adding a couple of words, are not enough to evade the issue.

You can read something on a website, then think about it and express it in your own words. But cut and paste? Nope.

Even paraphrasing–i.e. changing most of the words without changing the meaning–would be plagiarizing unless you properly cite it as a source.

Your paper should be your work. That means both your ideas and your words. You can include a few quotes and (properly cited) summaries of information you find in other sources, but they should be used to back up YOUR argument, which should be unique and not just repeating what you find in books and on websites.

If there is any doubt, just cite the original work as a reference.

This does not mean you can’t embed others work in yours for educational purposes, but you must mark and cross reference every quotation with a bibliography giving appropriate credit.