A deficiency is a lack or shortage of something that's needed. When your body has a vitamin deficiency, you don't have enough of a particular vitamin to stay healthy. A calcium deficiency might make your bones weaker, while an iron deficiency could leave you feeling tired all the time.

The word applies beyond health too. A school might have a deficiency in art supplies, meaning they don't have enough paint, paper, or brushes for students to use. A chess player might have a deficiency in their endgame strategy, a weakness they need to work on improving.

Deficiency refers specifically to important things that are missing, where the shortage creates a real problem or holds something back from working properly. You wouldn't say there's a deficiency of video games in your house (though you might wish for more!), but you would say there's a deficiency of clean drinking water in a drought-stricken area.

Deficient is the adjective form: a diet deficient in protein, or knowledge deficient in key facts. When doctors test your blood, they're checking whether you're deficient in any crucial nutrients your body needs to function well.