what is icing?

when a player on a team,who is on defense,in front of their goaltender,shoots a puck untouched down to the other end of rink... in NHL,two players chase puck,and if offensive player touches puck first,in chase,it is called "icing the puck"... in some leagues,you do not have to chase the puck;it is called,"no touch icing"... if player shoots puck without crossing center ice line,it is also icing,too..