In legislative terms, a bill is a proposal
a. for any law.
b. that has reached committee for debate.
c. made in the house of representatives.
d. made in the senate.

A for any law. It is the term for a proposed piece of legislature that has not yet passed a vote. in the American system the executive must also sign the bill after it is voted upon for it to officially become law. Also in this system, a bill can be introduced in either house and then move to the other, or differing versions can be created in both houses that must then be synthesize into a final version to vote and present to the executive.