What is HP?

I recently got premium, and have made some upgrades. I'm enjoying the features, but I have a question:

What is HP?

On some of the upgrades it says things like +3 HP. What does HP stand for and what does it mean?

Thank you

HP=Health Points

HP = life
if you attack someone, your unit will reduce defenders hp as a function of your attack
if you defend against someone, your unit will reduce attackers hp as a function of your defence

Health points. Also known as hit points. When your HP goes to 0, you "die". In this case (atWar), each units has HP, which is 7 by default. When an unit has 0 HP, it dies.