Insufficient means not enough to meet a need or requirement. When something is insufficient, there isn't an adequate amount or it doesn't measure up to what's necessary.

If you study for only ten minutes before a big history test, that preparation is probably insufficient for you to do well. If a recipe calls for two cups of flour but you only have one cup, you have insufficient flour to make the recipe work. When a teacher says your essay has insufficient detail, they mean you need to add more information and examples to fully answer the question.

You might have insufficient time to finish all your homework if you spend too long playing video games first, or insufficient evidence to prove your brother ate the last cookie, even though you're pretty sure he did.

Insufficient is a formal, precise way to say something falls short. Scientists might say there's insufficient data to draw a conclusion. A judge might rule there's insufficient evidence for a case. It's a word that points to a gap between what you have and what you actually need.