A memorial service was an event held to remember a person or persons who had died.
After Miles O'Brien and Julian Bashir were believed to have been killed aboard a T'Lani munitions cruiser, Benjamin Sisko and Kira Nerys planned a memorial service to be held a day later. Ambassadors E'Tyshra and Sharat later told Commander Sisko a memorial service was to be held on T'Lani Prime. (DS9: "Armageddon Game")
In an alternate 2372, a memorial service was held on the Promenade for Benjamin Sisko who was believed to have been killed, although he had actually suffered temporal displacement. Many people came forward to speak about him, including Major Kira. Jake Sisko did not, as he felt that "no matter what I said about him, I'd be leaving so much more out... and that didn't seem right." (DS9: "The Visitor")
The crew and civilians of Deep Space 9 held a memorial service for Morn in 2374. As per Lurian custom, the guests brought food and drink to sustain Morn in the afterlife. (DS9: "Who Mourns for Morn?")