The most common Korean word for “letter” is 편지 (pyeonji). There is one more word with a similar meaning — compare them below. 「letter」는 한국어로 편지.
letter
TOPIK 1 · noun
letter; character
TOPIK 2 · noun
Words and levels: TOPIK I essentials (National Institute of Korean Language standard curriculum, KOGL Type 1) · examples: Cheesepotato