A glassful is the amount of liquid that fills a drinking glass. When a recipe calls for a glassful of milk, it means however much milk your glass holds when filled. When you're thirsty after playing outside and gulp down a glassful of water, you've drunk the entire contents of one glass.

The word is useful because glasses come in different sizes. A small juice glass might hold six ounces, while a tall glass might hold twelve. When someone says they drank three glassfuls of lemonade, they mean they filled and emptied their glass three times, whatever size that glass happened to be.

Notice that glassful takes an s in the plural: glassfuls, not glassfuls spelled with three s's. You might drink two glassfuls of orange juice at breakfast. The word works like handful or cupful: it describes a quantity based on the container's capacity, not a precise measurement.