柱
Hashira
"Pillar" — the nine highest-ranked swordsmen of the Demon Slayer Corps. 柱 is the load-bearing column of a house, and also Japanese's counter for gods. VIZ keeps it romanized ("WATER HASHIRA: GIYU TOMIOKA"); Giyu insists he doesn't deserve the rank.
鬼殺隊
Demon Slayer Corps
The demon-hunting organization — literally the "demon-killing corps" — that Giyu serves as Water Hashira and Tanjiro joins on his advice.
水の呼吸
Water Breathing
Giyu's combat style — 全集中・水の呼吸, "Total Concentration: Water Breathing." Ten inherited forms; Giyu added an eleventh of his own.
凪
Dead Calm
The Eleventh Form Giyu invented — 凪, an everyday weather word for a windless, mirror-flat sea. A Japanese-made kanji (wind-frame + 止 "stop") and a two-mora name in a school of long ones. VIZ's DEAD CALM has its own Gem.