The practice of taking someone else's work or ideas and passing them off as one's own.
Taken from: http://www.oxforddictionaries.com/us/definition/american_english/plagiarism
Find on shelfCirculating - PN171 .F56 L56 2008
Find on shelfCirculating - PN171 .F56 N48 2010
These are great resources for learning about plagiarism and how to cite your research.
As a general rule, if you have used words, ideas, images, etc. in your research, that aren't your original words, ideas or images, it is best to cite your work. Penalties for plagiarism can be severe.