In memoriam is a phrase that's used to write about someone who has died. Your grandpa's online obituary might start with the heading In Memoriam.
In Latin, in memoriam means "in memory of." These words have been used to honor deceased people for generations; you'll see them carved into gravestone epitaphs in old cemeteries. It's also a common heading for a list of people who have died, which might be printed in a newspaper or college alumni magazine. If you're composing an obituary in memoriam of your beloved piano teacher, you'll write carefully, in a way that honors her wonderful life.