I guess "prevening" is not even an official word, but it has been created by Sheldon, one of the characters of "The Bing Bang Theory". Here is part of the dialogue in the episode where he "explains" it:
-Wolowitz: "So, listen, what are you doing tomorrow afternoon?" -Sheldon: "Be more specific." -Wolowitz: "4:30." -Sheldon: "That's not afternoon. That's prevening." -Wolowitz: "What?" -Sheldon: "It's a time of day I invented. It better defines the ambiguous period between afternoon and evening, prevening."