Exertion is the effort you put into physical or mental work. When you run up three flights of stairs, the burning in your legs and the heaviness in your breathing come from physical exertion. When you concentrate hard on a difficult math problem, working through each step carefully, that's mental exertion.

The word captures that feeling of pushing yourself: exertion means you're working hard enough that your body or mind notices. Light activities like walking to class require little exertion, but sprinting across the playground requires significant exertion. Solving a simple addition problem takes minimal mental exertion, while writing a five-paragraph essay demands considerable mental exertion.

People often talk about exerting yourself or exerting effort. An athlete exerts tremendous effort during practice. A musician exerts intense concentration while learning a challenging piece. After serious exertion, you might feel tired, but you've also accomplished something worth doing.

Notice that exertion isn't quite the same as struggle: you can exert yourself successfully and feel great afterward. Exertion is simply the energy and effort you invest in getting something done, whether that's climbing a tree, solving a puzzle, or helping move furniture.