To expend means to use up or spend something, especially energy, effort, or resources. When you expend energy running around at recess, you're using up your body's fuel until you feel tired. When a scientist expends years of work on a difficult problem, she's investing tremendous time and effort into finding an answer.

The word often appears when discussing how much of something gets used. A video game character might expend health points during a battle. A rocket expends fuel as it climbs toward space. Your parents might talk about expending money on home repairs or a family vacation.

Expend suggests a deliberate use of limited resources. You don't expend air by breathing or expend sunlight by going outside, because those things aren't limited in the same way. But you do expend the battery in your tablet, the ink in your pen, or your patience while waiting in a long line. The related noun is expenditure: the amount of something used or spent. A team might review their expenditure of time and money before starting a big project, making sure they have enough of both to finish what they started.