when

adverb

1. At what time? At which time? Upon which occasion or circumstance? Used to introduce direct or indirect questions about time.

   “When will they arrive?”

2. Used after a noun or noun phrase in isolation to express impatience with an anticipated future event.

   “The site's all bugged. Fix when?”

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

   “He's mister high and mighty now, but I remember him when.”

4. At which, on which, during which: often omitted or replaced with that.

   “That was the day when the Twin Towers fell.”

conjunction

1. At (or as soon as) that time that; at the (or any and every) time that; if.

   “Pavlov's dogs salivate when [i.e. at any and every time that] they hear a bell.”

2. During the time that; at the time of the action of the following clause or participle phrase.

   “They dream when [i.e. during the time that] they sleep.”

3. At what time; at which time.

   “I am here till Friday, when [i.e. at which time] I leave for Senegal.”

4. Since; given the fact that; considering that.

   “I don't see the point of putting up Christmas decorations when I am the only person who is going to see them.”

pronoun

1. What time; which time.

   “Since when do I need your permission?”

noun

1. The time at which something happens.

   “A good article will cover the who, the what, the when, the where, the why and the how.”

interjection

1. That's enough: a command asking someone to stop adding something, especially an ingredient or portion of food or drink; used in, or as if in, literal response to ‘Say when’.

2. Expressing impatience.