Steadfast

The Japanese word 不動心 meaning "Steadfast" also translates as "indomitable spirit; immovable heart" and is composed of the kanji 不 (read fu) meaning "not", 動 (read dou) meaning "move", and 心 (read shin) meaning "heart; mind; spirit". Fudoshin embodies the ideal that one should be "unshakable" in any situation regardless of the difficulty. The colloquial "cool under fire" immediately comes to mind as a suitable translation.