Closure is the end or the closing down of something. It can be physical — like the closure of your local library — or emotional, like the closure you experience when you finally come to terms with the end of a romance.
Closure comes from the Latin claus ("shut"), and it has many different shades of meaning. A road closure blocks that road from traffic. If you find closure after an emotional hardship, you're ready to move on. An obstruction in a small passage, like a pipe, is also a closure, as is the button on your sweater. In debate, closure (usually cloture), stops debate and starts the vote.