Stackable means designed to fit neatly on top of each other without tipping or wasting space. Stackable chairs in a classroom can be piled up and stored in a corner when you need more room. Stackable storage containers have flat tops and bottoms so they form stable towers in a closet or pantry.

Good stackable design isn't accidental. Engineers and designers create matching edges, flat surfaces, and balanced shapes so items stay steady when stacked. Plastic cups at a fast-food restaurant are stackable, fitting together to save counter space. Shipping crates are stackable so warehouses can store more goods in less space.

The word can also describe things that combine or accumulate. In video games, stackable bonuses or power-ups can add together, making your character stronger with each one you collect. Some discounts are stackable, meaning you can use more than one at checkout.

When something isn't stackable, you notice immediately: round-bottomed bowls wobble dangerously in a stack, oddly shaped boxes leave wasted gaps, and you end up needing much more storage space than you should.