The practice of taking someone else's work or ideas and passing them off as one's own.
Taken from the Oxford Dictionary online
Doing Honest Work in College
Circulating - PN171 .F56 L56 2008
The Complete Guide to Referencing and Avoiding Plagiarism
Circulating - 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.