When Definition

wĕn, hwĕn

adverb

At what time?

When did they leave? he asked when he should go.

Webster's New World

On what occasion or under what circumstances?

When do you double the final consonant?

Webster's New World

At what point?

When shall I stop pouring?

Webster's New World

At an earlier time and under different, often less favorable, circumstances.

I knew him when.

Webster's New World

(interrogative) Used to introduce questions about time.

When will they arrive?

Wiktionary

conjuntion

At the time that.

When we were at college.

Webster's New World

At what time or point.

They told us when to eat.

Webster's New World

At which.

A time when people must speak out.

Webster's New World

As soon as.

The runners started when the gun went off.

Webster's New World

At whatever time; whenever.

She smiles when you praise her.

Webster's New World

Synonyms:

pronoun

What time.

Until when will you stay?

Webster's New World

Which time.

We came a week ago, since when we've been very busy.

Webster's New World

noun

The time or moment (of an event)

The when and where of his arrest.

Webster's New World

interjection

That's enough, a command to stop adding something, especially an ingredient of food or drink.

Wiktionary

Other Word Forms of When

Noun

Singular:

when

Plural:

whens

Origin of When

From Middle English when(ne), whan(ne), from Old English hwenne, hwænne, hwonne (“when"), from Proto-Germanic *hwannÄ“ (“at what time, when"), from Proto-Indo-European *kÊ·o-, *kÊ·i- (“interrogative base"). Cognate with Dutch wanneer (“when") and wen (“when, if"), German wann (“when") and wenn (“when, if"), Gothic 𐍈𐌰𐌽 (Æ•an, “when, how"), Latin quandō (“when"). More at who.

From Wiktionary
Interjection sense: a playful misunderstanding of "say when" (i.e. say when you want me to stop) as "say [the word] when".

From Wiktionary
Middle English from Old English hwenne kwo- in Indo-European roots

From American Heritage Dictionary of the English Language, 5th Edition