What is Plagiarism?

Plagiarism, we often hear this word spoken by our professors. When asked to do term papers, reaction papers, etcetera. Our professors strictly instruct us to avoid plagiarism, for it is a capital sin. But what does this word really mean?
I have conducted a little research about plagiarism, and found different definitions. Merriam Webster’s Dictionary defined plagiarism to be the “usage of words or ideas of another as if these words or ideas were your own.” And Grolier New Webster’s Dictionary defined plagiarism “the use and pass off of someone else’s ideas, inventions, writings, etcetera as one’s own.” Though the said definitions came from two different sources their point is exactly the same. Plagiarism is the stealing of ideas, inventions, and writings of other people, proclaiming that the crook came up with those ideas, inventions, and writings, using other’s work without giving credit to their efforts, or even mentioning the name of the person who originally came up with the idea. At least this is what I’ve understood from the definitions I have quoted above. The reason we were given access to other people’s writings, inventions, and concepts is to inspire us or inform us. They spread their work to give help to others, and to be appreciated for their hard work. Thus, when we quote their words or create inventions due to their inspiring discoveries, it is quite ethical to acknowledge, thank, or even just simply state their name for recognition. It must have been a delightful feeling for the inventors, and writers to have their work be appreciated by other people. Come to think of it, it is sort of a give and take relationship.
Plagiarism is not different from stealing. Once you plagiarize, you are a thief. Now tell me, are you a criminal?