This is a very hurtful old name for Down's syndrome — the condition, not the person. Do not use it. A doctor named John Langdon Down coined it in the 1800s, from Mongol plus -ism, because he thought the faces of people with the condition looked like those of Mongol people. That mixed a people with a medical condition, and it caused long harm. English has put the word away; doctors and teachers say Down's syndrome. Though actually, the nearby word mongol was the old insult for the person, while mongolism named the condition. Both are forbidden now. You may still find mongolism in very old papers. If you see it, you are not looking at a current word.

（極冒犯，勿用）唐氏症的舊稱「蒙古症」；請說「唐氏症」（對岸多說「唐氏綜合徵」）