Deficient means lacking something important or not having enough of what's needed. When something is deficient, it's missing a necessary part or falls short of what it should be.

A diet deficient in vitamin C can make someone sick, because the body needs that vitamin to stay healthy. A report deficient in details leaves out important information that readers need to understand the topic. If a building's fire safety system is deficient, it doesn't meet the standards required to keep people safe.

The word often appears in more formal contexts, like when doctors talk about vitamin deficiencies or when inspectors find that equipment is deficient. You might hear that a student is deficient in certain math skills, meaning they haven't yet learned what they need to know to move forward.

Notice that deficient suggests something measurable: there's a standard or requirement, and whatever is deficient doesn't measure up. It's different from just “not enough.” When something is deficient, we can usually point to exactly what's missing or inadequate. A glass that's half full of water isn't deficient unless you need a full glass, but a structure deficient in support beams is missing something it needs to be safe and complete.