Plagiarism is taking someone else work or ideas and using them as your own without giving the source credit.
Even if unintentional it is still plagiarism. When in doubt better not use that phrase or idea. It is perfectly ok to cite "too much" than to not cite enough and be accused of cheating!
Here is an official dictionary definition from Merriam-Webster:
transitive verb : to steal and pass off (the ideas or words of another) as one's own : use (another's production) without crediting the sourceintransitive verb : to commit literary theft : present as new and original an idea or product derived from an existing source
You can avoid plagiarism by always giving credit and citing the sources you are using.
When paraphrasing be sure you are indeed paraphrasing and don't forget to give credit to the author of the idea.
If what the author wrote can't be said better be sure to do an exact quotation and provide the required information guided by your citation style.
Keep in mind that:
You can never cite too much. It is better to be accused of citing too much than of not citing enough!
When in doubt of where "it" (the idea, paraphrase or quotation) came from leave it out. Find another sourcre instead.
You can only cite what you have read.
Five Types of Plagiarism:
1. "Copy & Paste Plagiarism"
"Any time you lift a sentence or significant phrase intact from a source, you must use quotations marks and reference the source."
2. "Word Switch Plagiarism"
"If you take a sentence from a source and change around a few words, it is still plagiarism. If you want to quote a sentence, then you need to put it in quotation marks and cite the author and article. But quoting Source articles should only be done if what the quote says is particularly useful in the point you are trying to make in what you are writing." In many cases, paraphrasing and then citing the original sources is a better option.
3. "Style Plagiarism"
"When you follow a Source Article sentence-by-sentence or paragraph-by-paragraph, it is plagiarism, even though none of your sentences are exactly like those in the source article or even in the same order. What you are copying in this case is the author's reasoning style."
4. "Metaphor Plagiarism"
"Metaphors are used either to make an idea clearer or give the reader an analogy that touches the senses or emotions better than a plain description of the object or process. Metaphors, then, are an important part of an author's creative style. If you cannot come up with your own metaphor to illustrate an important idea, then use the metaphor in the Source Article, but give the author credit for it."
5." Idea Plagiarism"
"If the author of the source article expresses a creative idea or suggests a solution to a problem, the idea or solution must be clearly attributed to the author. Students seem to have a hard time distinguishing author's ideas and/or solutions from public domain information. Public domain information is any idea or solution about which people in the field accept as general knowledge. For example, what a black hole is and how it is defined is general knowledge. You do not need to reference a general description of a black hole. The escape velocity of earth is also general knowledge and needs no reference. The distance to the center of the Galaxy is also general knowledge. However, a new idea about how to look for black holes or a new solution to a physics problem needs to be attributed to the authors. If you don't know what is accepted as public domain in a particular field, ASK."
Five Types of Plagiarism Taken From:
Barnbaum, C. “Plagiarism: A
Student's Guide to Recognizing It and Avoiding It.”
University. http://www.valdosta.edu/~cbarnbau/personal/teaching_MISC/plagiarism.htm (Accessed 15 September 2008).
Liles, Jeffrey A. and Michael E. Rozalski. “It's a Matter of Style: A Style Manual Workshops for
Preventing Plagiarism.” College & Undergraduate Libraries, 11 (2), 2004, p. 91-101.