When you breathe hard and fast, you pant. It's perfectly normal to pant after you finish running a marathon — or even after chasing your runaway dog through the neighborhood.
You might pause at the top of a hill to pant and catch your breath before you continue on your hike. That loud, gasping breath itself is also a pant — but when this noun is plural, it means trousers, slacks, or jeans. Be careful, though — in parts of the UK, pants are primarily women's undergarments. Any pants you wear on you body come from pantaloons, which in the 1600s meant tights.