noun
piled stones; stacked stones
Refers to stones that have been intentionally piled or stacked, often for a purpose such as a marker, monument, or simple structure.
登山道には積み石がいくつもあった。
There were many piled stones along the mountain trail.
noun
Used for a stone that serves as a foundation or cornerstone in construction, or metaphorically as a fundamental element.
この建物の積み石は百年以上前のものだ。
The foundation stones of this building are over a hundred years old.
Compound of 積み (tsumi, 'piling', the continuative form of 積む) and 石 (ishi, 'stone'). The literal meaning is 'piled stone'.