The word 妖精, pronounced YOUSEI (ようせい) is very important in Berserk. In Japanese, it designates "fairies, elves and sprites". In Berserk specifically, it is associated with the term "elf", which is typically applied to it using the furigana エルフ (which indicates how it should be pronounced).
The term エルフ actually appears first in the series, it's used as standalone Katakana to refer to Puck throughout the Black Swordsman arc. 妖精 first appears during the Golden Age arc, when Judo gives Casca some elf dust to heal Guts' wounds after the hundred men battle.
The words 妖精 and エルフ take on a larger meaning as the series develops, eventually encompassing different types of elves, among which are dwarves. We see them in Elfhelm, which is written 妖精郷 (hometown of elves) with the furigana エルフヘルム attached to it.