What do economists mean by “diminishing returns” to an input?

What do economists mean by “diminishing returns” to an input? What causes diminishing returns? Have you ever observed this principle at work in a job you’ve had or in the real world? Discuss the long and short run impact of this concept on the firm.

Your initial post should be substantial and answer the questions in detail.  Also, you must post at least TWO replies to others’ posts.  Your replies must be a minimum of 4 sentences long and, again, must be meaningful and detailed.