Final answer:
A piece of legislation being debated and amended in the House of Representatives is referred to as a 'marked up' bill during its markup phase. This is prior to being voted on and potentially sent to the Senate.
Step-by-step explanation:
When legislation is being debated in the House of Representatives and is amended, the term that refers to it at this point in its development is a marked up bill. This stage, known as the markup phase, involves an amending and voting process within the committee or subcommittee. If a committee decides to advance the bill with or without amendments, it is printed and sent to the House floor after first going through the House Committee on Rules which establishes the rules of debate. Once on the floor, the bill is debated, further amendments can be added, and eventually, a vote is taken to determine if the bill will advance to the Senate.