Mental Illness
Mental Disorder
noun
精神 is your spirit or mind, the non-physical part of you. When it's sick, it's a mental illness or mental disorder.
Note that the term 精神病, when used on its own, carries a stigma and is considered a bit disrespectful and outdated.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Will help if you know 精神 already, though.
父は精神病をわずらって入院しています。
My father is suffering from a mental illness and is in hospital.
精神病と身体の健康は深く関係しています。
Mental illness is closely related to physical health.
猫が急に変な動きをしていて、「これって精神病かな？」と思っちゃった。
My cat started acting weird, and I wondered, "Is this a mental illness?"