Critical means extremely important or necessary. When doctors say a patient is in critical condition, they mean the person's health is at a dangerous point where things could go very wrong. When your teacher says reading the instructions is critical to completing the lab experiment safely, she means you absolutely need to do it.

Something critical is so important that without it, failure becomes likely or even certain. The final seconds of a tied basketball game are critical moments where one play determines who wins. A critical component of an engine is a part that makes the whole thing stop working if it breaks.

The word also means expressing careful judgment about something's strengths and weaknesses. A critical analysis of a book examines both what works well and what doesn't. Critical thinking means carefully evaluating information instead of just accepting everything you hear. When someone offers critical feedback on your essay, they're pointing out problems that need fixing, but they might also note what you did well. This kind of feedback helps you improve.

People sometimes say they're critical of something when they disapprove of it or see serious flaws. A scientist might be critical of a research method that doesn't test things properly. This doesn't always mean being negative: good critical thinking requires fairness and honesty about both strengths and weaknesses.