In denial is a state of mind in which someone is unwilling or unable to accept a truth or reality that is psychologically painful or difficult to bear. It is a defense mechanism that helps people cope with difficult emotions and experiences by temporarily blocking them from awareness.
An example of someone who acts in denial is someone who continues to smoke cigarettes despite having been diagnosed with lung cancer and told by their doctor that they need to stop smoking in order to live. The person may be unable or unwilling to accept the reality of their condition and the gravity, instead continuing to engage in the behavior that is ultimately harming them.