The Charm of Amaterasu is a major Defense charm, named for the kami of the sun, in Ghost of Tsushima. Every kill heals you by a moderate amount. You get it by honoring the Golden Summit Shrine in Izuhara.
Climb to the Golden Summit Shrine, south of the Golden Temple in Ariake Prefecture, and honor it at the top. You can reach it without the grappling hook.
It keeps you topped up in long fights against groups, since each body you drop gives health back.
Aaruul
Aaruul is one of the 50 Mongol Artifacts, pieces of the invaders' culture and warfare you can study. It sits at a camp south of Izuhara Clearing in Izuhara, and finding 20 artifacts counts toward the Know Your Enemy trophy.
Adachi Dragon Mask
Adachi Dragon Mask is a New Game+ mask sold by Baku the Voiceless for 5 Ghost flowers. It does not exist in a first playthrough.
Adak Tree
Adak Tree is one of the 50 Mongol Artifacts, pieces of the invaders' culture and warfare you can study. It sits at Yoichi's Crossroads in Izuhara, and finding 20 artifacts counts toward the Know Your Enemy trophy.
Adventurer's Fortune
Adventurer's Fortune is a dye for Traveler's Attire. It costs 10 flowers at the Izuhara merchants. Like every dye it only changes colours.