What is Plagiarism?

What is plagiarism? This question was discussed in one of my recent classes. I find the Merriam-Webster definition - "present as new and original an idea or product derived from an existing source" - to be the most useful.

It's an practical question, because plagiarism is all around us. 

1. At this moment, I have the Verve running on my iPod. While working on my homework, "Bittersweet Symphony" played. This is the 1997 hit that was built off a sample of an orchestral version of the Rolling Stones' "The Last Time", which was 'inspired' by "This May Be The Last Time" by The Staple Singers, which has roots in a traditional Gospel song. Next song on my play list  was Jay-Z rapping over "Bittersweet Symphony" in a track titled "Brush Your Bitter Sweet Shoulders Off". Dizzy yet?
   Five generations and only the last one gave proper credit... (http://www.songfacts.com/detail.php?id=478)
2. A few weeks ago, I had lunch with Greg Pyper. I've worked with Greg for a few years, since he was with Marcum & Smith. He now works at a coaching company called InsideOut.
   He asked what coaching model we use and I told him we use the GROW model. He grimaced as he informed me that the founder of his company was part of the group that 'created' the GROW model in the 70's. No one copyrighted it, and now it appears all over the Internet, often unattributed. While it may not technically be 'public domain' (lawyers are trying to figure that out, I guess), it might be common knowledge at this point. What to do?

These are two cases that are just the tip of a massive (and growing) iceberg.
Scenarios like these undermine the credibility of the creator, the adopter, and even of the idea itself.

Knowledge and wisdom are built on the past and taken forward.
That which is built on the past should be honored and then challenged. 

That which is new should be challenged and then, if it passes the test of time, honored. 

To do this, we must know the difference.

What can be done? I think you have to honor the source, both intellectually and in a sense, spiritually. 
That is, you have to know your own voice and know when you're not speaking in your own voice. 
Of course, colleagues can be a big help in pointing that out, as well.

Thoughts?