The Oxford English Dictionary defines plagiarism as follows: "The action or practice of taking someone else's work, idea, etc., and passing it off as one's own; literary theft."
The idea of plagiarism as theft is an important one in American academic culture; it stems from the concept of intellectual property.
Western cultures, such as those of the United States, Canada, the United Kingdom, and Australia, commonly value intellectual property. Intellectual property is the idea that a person or organization can own a idea, a phrase, an essay, a song, a particular photograph, etc. Ideas can be owned even if the owner decides to make the idea freely available on the Internet. For instance a band might decide to release a video of a particular song on YouTube. Despite making that video available the band still owns the content, and people finding that video on YouTube cannot claim ownership or use that video any way they please. If you took a video from YouTube, put it on your own web site and tried to sell it, you would find yourself in court defending yourself against a law suit.