sudden

UK: /ˈsʌd^ən/ US: /ˈsʌdən/ ,(sud′n)

WordReference Random House Learner's Dictionary of American English © 2025

sud•den /ˈsʌdən/   adj. 

1. happening, coming, made, or done quickly or unexpectedly:
   a sudden attack.

adv. 

2. [Literary.] suddenly.

Idioms

3. Idioms all of a sudden, without warning;
   unexpectedly:
   All of a sudden we were surrounded.

sud•den•ly, adv.: 

She lunged at him suddenly.

sud•den•ness, n. [uncountable]

WordReference Random House Unabridged Dictionary of American English © 2025

sud•den  (sud′n), adj. 

1. happening, coming, made, or done quickly, without warning, or unexpectedly:
   a sudden attack.
2. occurring without transition from the previous form, state, etc.;
   abrupt:
   a sudden turn.
3. impetuous;
   rash.
4. [Archaic.] quickly made or provided.
5. [Obs.] unpremeditated.

adv. 

6. [Literary.] suddenly.

n. 

7. [Obs.] an unexpected occasion or occurrence.
8. all of a sudden, without warning;
   unexpectedly;
   suddenly.

Also, on a sudden. 

• Latin subitāneus going or coming stealthily, equivalent. to subit(us) sudden, taking by surprise (see subito) + -āneus composite adjective, adjectival suffix, equivalent. to -ān(us) -an + -eus -eous
• Middle French
• Middle English sodain (adjective, adjectival and adverb, adverbial) 1250–1300

sud′den•ly, adv. 
sud′den•ness, n. 
Synonyms

• 1, 2. unforeseen, unanticipated.
  Sudden, unexpected, abrupt describe acts, events, or conditions for which there has been no preparation or gradual approach.
  Sudden refers to the quickness of an occurrence, although the event may have been expected:
  a sudden change in the weather.
  Unexpected emphasizes the lack of preparedness for what occurs or appears:
  an unexpected crisis.
  Abrupt characterizes something involving a swift adjustment; the effect is often unpleasant, unfavorable, or the cause of dismay:
  He had an abrupt change in manner. The road came to an abrupt end.

Antonyms

• 1, 2. gradual, foreseen.

Collins Concise English Dictionary © HarperCollins Publishers::

sudden / ˈsʌd^ən/ adjective

1. occurring or performed quickly and without warning
2. marked by haste; abrupt
3. rash; precipitate

noun

1. an abrupt occurrence or the occasion of such an occurrence (in the phrase on a sudden)
2. all of a sudden ⇒ without warning; unexpectedly

Etymology: 13th Century: via French from Late Latin subitāneus, from Latin subitus unexpected, from subīre to happen unexpectedly, from sub- secretly + īre to go

ˈsuddenness noun