再び is an adverb meaning 'again' or 'once more.' It is more formal and literary than また, carrying a nuance of repetition after an interval or a deliberate return to a previous state. You will often see it in written announcements, news, or speeches, but it can also be used in everyday conversation when you want to sound a bit dramatic or serious.

Use it directly before the verb or phrase it modifies: 再び + verb (dictionary or past form depending on context). Unlike また, it does not combine with casual sentence endings as freely; it sounds odd in very informal chat. For example, 'I'll see you again' could be 再びお会いしましょう, but in casual speech you'd say また会おう.

Note that 再び often implies a second occurrence of something that had previously happened, but it can also be used for a fresh attempt. It is not used for habitual repetition (for that, use また or もう一度).