According to the Merriam-Webster Online Dictionary, to "plagiarize" means
In other words, plagiarism is an act of fraud. It involves both stealing someone else's work and lying about it afterwards.
All of the following are considered plagiarism
turning in someone else's work as your own
copying words or ideas from someone else without giving credit
failing to put a quotation in quotation marks
changing words but copying the sentence structure of a source without giving credit
using ideas from a source for the majority of your work, whether you give credit or not
buying a paper online
reusing a paper written for another class (unless you have permission from your current professor)