deceased parent
A respectful and somewhat formal term for a parent who has passed away. Often used in contexts of remembrance or legal matters.
I put my hands together in prayer before the portrait of my deceased parent.
I still cherish the teachings of my late parent.
亡母 specifically means 'deceased mother', while 亡き親 can refer to either parent.
Compound of 亡き (naki, 'dead, deceased', the classical attributive form of 亡し) and 親 (oya, 'parent').