a.

1. 必要的，必需的[（+for/to）]
   Oxygen is necessary for life.

   氧气是生命所必需的。

   It is necessary that he be sent there at once.

   有必要马上派他到那里去。

2. 必然的，无法避免的
   Poor health is a necessary result of over-exertion.

   体弱是过度劳累的必然结果。

n.

1. 必要的物品；必需品[P1]
   She has enough money to obtain the necessaries of life.

   她有足够的钱购买生活必需品。

adj.

1. required to be done, achieved, or present; needed.
2. inevitable:
   a necessary consequence.
   ▸Philosophy  inevitably resulting from the nature of things, so that the contrary is impossible.
   ▸Philosophy  (of an agent) having no independent volition.

n.

1. (necessaries) the basic requirements of life, such as food and warmth.
2. (the necessary) informal the action or item needed.
   ▸Brit. informal the money required.