What is waste?

Although the word ‘waste’ can be used in different respects, when I hear the word, I think of anything that could serve a purpose, but is either thrown away or disregarded. The prime example of this would be uneaten food. I rarely throw out my food. If I am finished eating it and there is more left, I put it in the refrigerator and eat it later. However, this is not the only form of waste. If I am finished with my clothing, I donate them to charity. I hate to see people wasting things that could be used by another, because there are so many people in the world who do not have either the money or the opportunity for such things. I always wonder why people just throw out things they no longer want instead of giving them to someone who does, but I am unable to arrive at an answer.