means The broken, decayed remains of buildings or structures that have been damaged by age, war, or neglect.

from From Latin ruina, 'a collapse, a falling down,' which comes from the verb ruere, 'to rush, fall, tumble down' — the same restless root that gives us 'rush.' English took it in through Old French ruine in the late Middle Ages. So at its heart, a ruin isn't just something old; it's something caught mid-fall, frozen in the act of coming down.