To stumble means to trip or nearly fall while walking or running, often catching yourself at the last moment. You might stumble over a tree root on a hiking trail, your foot catching unexpectedly and sending you lurching forward before you regain your balance. Sometimes you stumble because you're not paying attention, sometimes because the ground is uneven or something's in your way.

The word also describes making mistakes or struggling with something you're trying to do. A student might stumble through a presentation when they haven't prepared well, pausing awkwardly and losing their train of thought. A musician might stumble over a difficult passage in a song, missing notes or losing the rhythm.

When you stumble upon or stumble across something, you discover it by accident, like finding an old baseball card collection while cleaning the attic or stumbling upon an interesting book at the library while looking for something else entirely. This kind of stumbling can lead to wonderful surprises.

People also use stumbling block to describe an obstacle that makes progress difficult. Learning multiplication might be a stumbling block for a student who finds math challenging, but with practice, they can get past it and keep moving forward.