1. 必然;必要
    The necessity of something is the fact that it must happen or exist.

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    e.g. There is agreement on the necessity of reforms...
           对改革的必要性已经达成了共识。
    e.g. As soon as the necessity for action is over the troops must be withdrawn...
           一旦军事行动的必要性不复存在，军队就必须撤出。

2. 必需品;必不可少的事物
    A necessity is something that you must have in order to live properly or do something.

    e.g. Water is a basic necessity of life.
           生命离不开水。
    e.g. ...food, fuel and other daily necessities.
           食物、燃料和其他日常必需品

3. 不可避免的情况(或行动)
    A situation or action that is a necessity is necessary and cannot be avoided.

    e.g. The President pleaded that strong rule from the centre was a regrettable, but temporary necessity.
           总统辩解说，实行高度的中央集权虽不尽如人意，但暂时来说却是必要的。