Porosity is the quality of being porous, or full of tiny holes. Liquids go right through things that have porosity.
Go back far enough and you’ll find that porosity stems from the Greek word poros for "pore," which means “passage.” So something with porosity lets things through. Most pants have porosity, as you unfortunately notice when you spill soup on them — the soup goes right through. On the other hand, a raincoat has no porosity because it's waterproof. If your roof is leaking during the rain, it has some porosity, which is a problem. You can remember porosity by imagining liquids pouring through things.