Question on the game term 'Stack'

I've read a lot of threads in this forum, and I found a mentioning of 'stack'. For example 'tenryu stack'.
Anybody can help me out?

 

Tenryuu ransei has 4 stages of attacks represented by stacks. The first stack is the initial skill which cost you 300mp, the 2nd, 3rd and 4th attack will have gradual increase in damage and animation speed with the cost of 100mp. After 4th attack, which is fully stacked, you will have the max damage of tenryuu, you can do it till the duration of tenryuu buff runs out, or you can renew your tenryuu buff by using zantei settetsu's counter attack or magadachi in which will reset the buff time to 30 s(iirc)

 

When activated, Tenryu has 4 stack, each use of it will increase 1 stack.
Your dmg and motion speed increases with each stack, maxing out at 4.
1st time using tenryu cost 300mp, the cost then goes down to 100mp as long tenryu is still active

 

So that's what it is. Thanks for the great input!

 

the damage only increases until the 3rd stack. 3rd stack and 4th stack tenryus do the same dmg