To exert means to put forth effort or to use strength, power, or influence to make something happen. When you exert yourself while running up a steep hill, you're pushing your body hard. When a coach exerts her authority to settle an argument between teammates, she's using her position to create order.

The word captures that sense of deliberate effort or force being applied. A scientist might exert pressure on a sealed container to test its strength. A parent might exert influence over a school decision. You exert mental energy when concentrating hard on a difficult problem. Athletes exert physical power during competition.

Exertion is the noun form. After a day of heavy exertion, like helping a friend move furniture or playing several soccer games in a row, you feel tired because you've used up your energy. The amount of exertion required varies: solving a crossword puzzle requires mental exertion, while climbing a mountain demands serious physical exertion.

Notice that exerting yourself isn't the same as just being busy. Scrolling on a phone keeps you occupied but doesn't require much exertion. Real exertion means actively pushing yourself, applying force, or working to achieve something that takes genuine effort.