蓄積
ちくせき · chikuseki · JLPT N1 · noun
accretion
🐿️🌰 蓄 = store up + 積 = pile up, so 蓄積 = store by piling up over time
Like a squirrel saving nuts all autumn for winter, 蓄積 means building up a growing stock bit by bit. The same word covers accumulating knowledge, money, or fatigue.