Hi Jims, thank you!
Except for the loop which is repeated again and again as the botton is down, I referred to a strange effect, something like a blinking, before every loop (blinking - LED changes from dark to bright - blinking - LED changes... etc. while the botton is kept down). It doesn't appear with both "for" loops (fade in and fade out)... But it's not very important, as I said it's only aesthetic... 
Anyway I think the variable you wrote is quite suitable!!!
What I want is... I push and release the botton, LED fades from dark to bright and remains turned on even if the botton is up.
LED can start lighting either when the botton is down and arduino reads a LOW (when the botton is released again, the LED is still on) or after the botton is released... it's the same... I'm very ignorant in matter of electronics and programming language, but i guess that Arduino has to read a LOW signal, anyway...
If you know how to translate your variable into code, may you add it please? Thank you again!