Negation takes on various forms in English, often not or its contraction -n't:
It was not feasible, in light of circumstances.
I didn't see it.
Sometimes it takes the form of an n- prefix (e.g. ever becomes never)
I have never eaten rabbit.
Other words which are commonly regarded as negations include hardly, barely, and scarcely; other negative prefixes include dis-, un-, and more.
Use the tag double-negation for questions regarding the use of two negations in a single clause.