Zenkai is the power boost that a Saiyan receives once he/she has experienced near death. In the Dragon Ball franchise, this can best be seen when Vegeta receives a large enough boost to over-power Zarbon, an enemy who had beaten him only a few episodes before.
In a much less literal definition, anyone can experience Zenkai. It's just when someone comes back stronger and better than ever from a bad L taken in life. We all have Zenkai.
Person 1: Damn bro I heard Laura rejected you the other day.
Person 2: It's all good man, my Zenkai is kicking in. I'll just hook up with her best friend.
It is a syndrome that affects students and it means that the student studies what has been studied in three or four months just few hours before the exam. This syndrome begins one or two days before the exam or in any means of transport while the student is literally on his way to sit for the exam.
Symptoms are “laughing for no reason accompanied by crying and increased hunger and thirst”..
( polyphagia , polydipsia , severe psychosis ).
The treatment usually consists of calming down the student and reminding him that the world is at the end a meter by meter.
(Doctors recommend serving him food he likes along with mint tea).
Friend: Do you want to hung out later?
Student: I can’t! I have an exam tomorrow! Just pray I survive this Dog’s zankation syndrome.
In Dragonball Z, it's the power boost a Saiyangets whenever he recovers from a mortal wound after losing a fight to an opponent.
Zarbon beat Vegeta to an inch of his life, but let him live. Vegeta, having lived the battle, got a zenkai and made a full recovery inside the healing tank of Frieza's ship. Zarbon was killed by Vegeta during their next encounter.