An adverb used to express strong negation: 'never' or 'by no means.' It must be used with a negative clause.
決して嘘をついてはいけない。
You must never tell a lie.
彼女は決して諦めない人だ。
She is someone who never gives up.
それは決して軽い問題ではない。
That is by no means a minor issue.