Something indescribable is too intense, extreme or unusual to describe. It's beyond words.
Have you ever noticed how some things are tough or impossible to put into words? Those things are indescribable. The feeling after you win the Olympic gold medal in swimming is probably indescribable. The astronauts who traveled to the moon had an indescribable experience. Some say an incredible athlete like Michael Jordan is indescribable. Of course, nothing is truly indescribable — people describe just about everything. Still, this is a great word for when you feel like words just aren't enough.