Aoristic

Aoristic \ey-uh-RIS-tik\ , adjective;
1. Indefinite; indeterminate
2. In grammar: A tense of the verb indicating past action without reference to whether the action involved was momentary or continuous

According to the Oxford English Dictionary, this word is Greek. The first attribution the OED lists for the first definition is from 1846 by George Grote: "In the genuine Grecian epic, the theme was an unknown and aoristic past."

I had never heard of aoristic in English grammar and it turns out there's a good reason for that: it doesn't exist. Aoristic is another term for perfective aspect, which is the opposite of the imperfective aspect. The imperfective aspect describes an action from a specific viewpoint - sort of like background information. The perfective aspect, on the other hand views a situation as a simple whole and describes a completed action. We can sort of express these in English; in a narrative the imperfective would describe the setting (The sun was shining. The birds were singing. etc.) while the perfective would describe what's happening in the foreground (The young lovers went to the park and had a lovely picnic.)

*Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Thursday, November 18