In Dragonball Z, it's the power boost a Saiyan gets 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.
1. A famous Unreal clan.
This clan had its humble beginnings in Unreal Tournament. Now it has 12 divsios, 3 of them being all women. Zenkai.com.
1. Zenkai pwns.
2. Zenkai is so huge now, I'll never get in.
3. Everyone dreams of being in the clan Zenkai.