essential

Compared meaning: needed, required for something (adjective).

比较义项：必要的，为达成某事或使某情况发生所需的（形容词）。

• necessary

  The neutral, general-purpose word for something needed to achieve a goal or make a situation happen; works in almost any context, formal or casual.

  语气中性、最通用的词，指为达成目标或使某种情况发生所需要的东西，几乎适用于任何正式或非正式场合。

  It's necessary to book a table in advance on weekends.

  周末最好提前预订餐桌，这是必要的。

• essential 本词

  Stronger than necessary; stresses that something is absolutely indispensable, so central that nothing can work or succeed without it.

  语气比 necessary 更强，强调某事物是绝对不可或缺的，核心到没有它就无法运作或成功。

  Regular exercise is essential if you want to stay healthy long-term.

  如果想长期保持健康，规律运动是必不可少的。

• requisite

  A formal, somewhat literary word, most often placed before a noun (the requisite documents/skills) rather than after 'is'; common in official contexts.

  书面语气较为正式，略带文学色彩，通常放在名词前使用（如 the requisite documents/skills），而不是放在系动词之后；常见于官方或公文语境。

  She finally gathered the requisite paperwork to renew her passport.

  她终于备齐了续办护照所需的必要文件。

• obligatory

  Means required by a law, rule or convention rather than by practical need; you must do it because you are told to, with no choice.

  表示因法律、规则或惯例而被要求，而不是因实际需要；必须去做是因为被规定如此，没有选择余地。

  Wearing a helmet is obligatory for all construction workers on site.

  工地上所有建筑工人都必须戴安全帽，这是强制规定。

Necessary, essential, requisite, and obligatory all mean needed or required. Necessary is the general word. Essential is stronger, meaning something is truly indispensable. Requisite is formal and literary, usually placed before a noun. Obligatory means required by a rule or law, not by real need.

这几个词都表示“必需、不可或缺”，但语气强弱和使用场合不同：necessary 最通用中性，essential 语气更强调绝对不可或缺，requisite 书面正式，obligatory 则强调因规则或法律而必须做，不是因实际需要。

Compared meaning: important (adjective).

比较义项：重要的（形容词）。

• essential 本词

  Strongest of the four: something essential is absolutely necessary, and things go wrong or fail without it. Common in formal, academic or health-related contexts, often followed by 'for' or 'to'.

  四词中语气最强：essential 表示绝对必要，缺少了它，事情就会出问题或失败。常用于正式、学术或与健康有关的语境，后面常接 for 或 to。

  Getting enough sleep every night is essential for good health.

  每天保证充足的睡眠对身体健康是至关重要的。

• primary

  Ranks one thing as the main or chief one among several possible options, rather than expressing sheer necessity. Common in formal or business contexts before nouns like goal, concern, source, cause.

  把某事列为几个选项中最主要、最首要的一个，而不是强调绝对的必要性。常用于正式或商务语境，后面接 goal、concern、source、cause 等名词。

  Improving customer service is our primary goal this year.

  提升客户服务是我们今年的首要目标。

• great

  Emphasizes grandeur, remarkable scale or lasting significance, often in historical, achievement-related or emotional contexts, rather than everyday necessity or ranking. Slightly formal or literary in tone.

  强调宏大、影响深远或历史意义，常用于历史、成就或情感相关的语境，而不是日常的必要性或排序。语气略显正式或带文学色彩。

  Marie Curie is remembered as one of the great scientists in history.

  玛丽·居里被后人铭记为历史上一位伟大的科学家。

• big

  The most casual and everyday of the four; used in ordinary speech for decisions, news or events that matter a lot, without the formality of essential, primary or great.

  四词中最口语、最日常的一个，用于口语中形容非常重要的决定、消息或事件，没有 essential、primary、great 那样正式的语气。

  Choosing a college was a big decision for our whole family.

  选择大学对我们全家来说是一个重大的决定。

Essential, primary, great, and big can all mean important. Essential means truly indispensable. Primary means the main one among options. Great suggests grandeur or lasting significance. Big is the most casual choice. Other senses, like essential 'basic' or big 'large', are not compared.

这四个词在表示“重要”时意思相近，但强弱和正式程度不同：essential 强调不可或缺，primary 强调首要、居首位，great 强调伟大或影响深远，big 则是口语中随意的“重要”。这里不比较 essential 表示“基本的、本质的”（如 essential oils）、primary 表示“最初的、小学的”（如 primary school）以及 great、big 表示“体积大”的义项。

Compared meaning: something needed or required (noun).

比较义项：必需的事物、必需品（名词）。

• necessity

  The most common, everyday word for something people cannot do without; often used in the phrase 'a basic necessity' and applies to both physical items and abstract needs.

  最常见、最口语化的词，用来指人们不可缺少的东西；常用于“a basic necessity”（基本必需品）这一说法，既可指具体物品，也可指抽象需求。

  For many rural families, a reliable car is a necessity, not a luxury.

  对许多农村家庭来说，一辆可靠的汽车是必需品，而不是奢侈品。

• essential 本词

  Usually used in the plural 'the essentials' for the basic items or steps needed for a particular task or situation; more casual and concrete than necessity or requisite.

  通常以复数形式“the essentials”出现，指完成某件事或应对某种情况所需的基本物品或基本要素；比 necessity 和 requisite 更随意、更具体。

  Pack the essentials — a map, water, and a flashlight — before you go hiking.

  去徒步之前，先把必需品——地图、水和手电筒——打包好。

• requisite

  Formal and somewhat old-fashioned; used with 'for' or 'of' to name something officially or formally required, typical of legal, academic or bureaucratic contexts.

  语气正式、略显书面化；常与 for 或 of 搭配，用来指官方或正式要求具备的东西，多见于法律、学术或行政等正式场合。

  Fluency in French is a requisite for this diplomatic position.

  流利的法语是担任这个外交职位的必需条件。

Necessity, essential and requisite all mean something needed. Necessity is the most common word for something people cannot do without. Essential is usually used as essentials, the basic items needed for a task. Requisite is formal, used with for or of in official contexts.

necessity、essential 和 requisite 在表示“必需的事物”时意思相近，但语气和正式程度不同：necessity 最常用、最口语化；essential 常以复数形式 essentials 表示某件事或某种情况所需的基本物品；requisite 则更正式、书面化，常与 for/of 搭配，用于申请、法律等正式场合。这里不比较 necessity 表示“必要性”的抽象义项（如 out of necessity），也不比较 essential 和 requisite 常见的形容词用法（如 essential skills、the requisite documents）。