Wiktionary
irrealis mood
n. (context grammar English) A category of grammatical moods that indicate that a certain situation or action is not known to happen, or have happened, as the speaker is talking.
Wikipedia
Irrealis mood
Irrealis moods ( abbreviated ) are the main set of grammatical moods that indicate that a certain situation or action is not known to have happened as the speaker is talking. This contrasts with the realis moods.
Every language has a formula for the unreal. Some languages have as many as five levels of "unreality".