periodic

Compared meaning: happening again and again over time (adjective).

比较义项：按固定或不固定的时间间隔重复发生的（形容词）。

• annual

  Annual specifies an event that happens exactly once every year, often tied to a fixed date or season; it cannot describe something repeating every day or week.

  annual 特指每年恰好发生一次的事情，通常与固定的日期或季节相关；不能用来描述每天或每周重复发生的事情。

  The magazine publishes an annual special issue every December focused on the year in review.

  这本杂志每年十二月都会出版一期年度特刊，回顾这一年的大事。

• daily

  Daily specifies an event that happens once every day, describing a regular daily habit or routine; it is not used for something happening once a year or once a week.

  daily 特指每天发生一次的事情，用来描述固定的日常习惯或惯例；不用于描述一年一次或一周一次的事情。

  He follows a daily medication schedule, taking pills right after breakfast each morning.

  他遵循每日服药计划，每天早餐后就吃药。

• periodic 本词

  Periodic describes something that repeats at intervals which may be regular or irregular and are usually unspecified, without naming a fixed unit like a year, week, or day.

  periodic 描述以间隔重复发生的事情，这些间隔可能规律也可能不规律，通常没有说明具体隔多久一次，不像其他三个词那样点明“年”“周”“日”这样固定的时间单位。

  Doctors recommend periodic eye exams every couple of years, even without symptoms.

  医生建议定期做眼科检查，大约每隔几年一次，即使没有症状也要检查。

• weekly

  Weekly specifies an event that happens once every week, often on a particular day; it contrasts with the shorter interval of daily and the longer interval of annual.

  weekly 特指每周发生一次的事情，往往在固定的某一天进行；与间隔更短的 daily（每天）和间隔更长的 annual（每年）形成对比。

  Our neighborhood hosts a weekly farmers market every Saturday morning downtown.

  我们社区每周六上午都会在市中心举办每周一次的农贸市场。

Annual, daily, weekly, and periodic all describe something that repeats, but the time unit differs. Annual means once every year. Daily means once every day. Weekly means once every week. Periodic means repeating at regular or irregular intervals, without naming a fixed time unit.

这四个词都表示按某种时间规律重复发生，容易被中国学习者混用；annual、daily、weekly 分别对应“每年”“每日”“每周”这样固定的时间单位，periodic 只强调重复发生、有规律或无规律，并不点明具体隔多久一次，使用时不能随意互换。这里不比较 annual 作名词时的“年鉴／一年生植物”义、daily 作名词时的“日报”义，以及 weekly 作名词时的“周刊”义。