[C] something that you need to have in order to live

必需品;

She saw books as a necessity, not a luxury.

她视书本为必需品而非奢侈品。

A car is an absolute necessity if you live in the country.

你要是住在乡下，绝对需要有辆汽车。

A lot of families cannot even afford to buy the basic necessities of life.

很多家庭甚至都买不起基本的生活必需品。

[U] when something is necessary

必要; 需要:

He emphasized the necessity for good planning and management.

他强调了做好规划和管理的必要性。

This illustrates the necessity of keeping accurate records of your work.

这说明有必要准确记录你的工作情况。

Many teachers are now questioning the necessity of formal exams.

现在许多老师都在质疑正式考试的必要性。

He only remained with the group out of necessity.

他只是出于需要才留在了那个小组。

economic / practical / political etc necessity

经济/实际/政治等需要

I'm afraid it's become a matter of economic necessity .

恐怕这已成了一个经济需求的问题。

[C] something that must happen, even if it is unpleasant

无奈的事物; 不好却必要的东西:

Taxes are a regrettable necessity.

税收是令人遗憾的无奈之举。

of necessity, [formal] used when something happens in a particular way because that is the only possible way it can happen

势必，必定:

Many of the jobs are, of necessity, temporary.

有许多工作岗位势必是临时性的。

necessity is the mother of invention, used to say that if someone really needs to do something, they will find a way of doing it

需要乃发明之母