The necessity of something is the fact that it must happen or exist.
例句
There is agreement on the necessity of reforms...對改革的必要性已經達成了共識。As soon as the necessity for action is over the troops must be withdrawn...一旦軍事行動的必要性不復存在，軍隊就必須撤出。
N-COUNT
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.
例句
Water is a basic necessity of life.生命離不開水。...food, fuel and other daily necessities.食物、燃料和其他日常必需品
N-COUNT
不可避免的情況(或行動)
A situation or action that is a necessity is necessary and cannot be avoided.
例句
The President pleaded that strong rule from the centre was a regrettable, but temporary necessity.總統辯解說，實行高度的中央集權雖不盡如人意，但暫時來說卻是必要的。