Goop is a playful word for any thick, sticky, slimy, or mysterious substance that's hard to identify or describe precisely. When you mix flour and water until it becomes a goopy mess, or when you find some unidentifiable goop stuck to the bottom of your shoe, you're dealing with goop.

The word captures that feeling of “I'm not sure exactly what this is, but it's definitely gross and sticky.” Scientists might use precise terms like “viscous substance” or “gelatinous material,” but goop is much more fun and honest. Kids encounter goop everywhere: the slime in a science experiment, the gunk that clogs a drain, mystery leftovers that have been in the fridge too long, or the gooey insides of a pumpkin at Halloween.

Goop isn't always bad, though. Some goop is useful, like hair gel or the thick hand soap that squirts out of dispensers. The word suggests texture more than value: something between liquid and solid, something that oozes, drips, or sticks. When something is goopy, it has that characteristic goop-like consistency. The word is informal and a bit silly, which makes it perfect for describing substances that are themselves kind of ridiculous.