(1)必要性;需要(great need)[U][N(for)，N+to-inf]：the～for providing protection for the convoys 为船队护航的必要性;As the weather is dry there is no～for a mac. 由于不下雨，不需要雨衣。There was no～at all for you to do that. 你完全没有必要这样做。Is there any～for another meeting? 还有必要再开会吗? There is no～to hurry. 没有必要匆忙。I don't think there is any～to bring him into the affair yet. 我想没有必要把他也卷进这件事。There is no～to wait any longer. 没有必要再等下去了。N～is the mother of invention. 需要是发明之母。What's the～of leaving so early? 这么早就离开有什么必要?〖同〗need，demand，pressure;

(2)必不可少的东西(something needed or essential)[C]：the～ies of life/of existence/for hiking/for a camping trip 生活必需品/生存所必需的东西/远足旅行所必需的物品/野营旅行必需品;For this job，a knowledge of Italian is a～. 做这项工作，必须懂意大利语。A water-bottle is an absolute～on this trip. 在这次旅行中水瓶是绝对必需的。〖同〗must;

(3)必然性(the power of natural law that cannot be other than it is)[sing]：historical/logical/physical～ 历史的必然性/逻辑的必然性/命运;Death is a～to life. 生命必然以死亡告终。

make a virtue of necessity出于无奈但尽力争取好的结果：After he lost all his money，he made a virtue of～and found a new and interesting life as a teacher. 他失去全部金钱之后，万般无奈作了教师，结果找到了一种新的有趣的生活。

of necessity 不可避免地;必然地：That discussion must of～be posponed for a while. 那个讨论必须推迟一段时间。