Permeable means allowing liquids or gases to pass through. A permeable material has tiny spaces or pores that let substances seep through it, like water soaking through a paper towel or air flowing through a screen door.

Think of a coffee filter: it's permeable to water but not to coffee grounds. The water passes right through while the grounds stay behind. Your skin is slightly permeable, which is why you can absorb some medicines through patches. A sponge is highly permeable, soaking up water into all its little holes.

Scientists and engineers care a lot about permeability. Geologists study how permeable different rocks are because water and oil flow through permeable rock layers underground. Farmers need to know if their soil is permeable enough to drain properly after rain but not so permeable that water drains away too fast for plants to use.

The opposite is impermeable, meaning liquids or gases can’t pass through. A raincoat is impermeable to water, keeping you dry. When describing ideas or organizations, people sometimes say they're permeable to new thinking, meaning they're open to outside influences. A classroom with permeable walls between subjects would be one where math ideas flow naturally into science lessons and vice versa.