Tafsir of Al-Furqan 25:39

وكﻼ ضربنا له اﻻمثال

And for each We put forward examples, meaning, `We showed them the proof and gave them clear evidence,' as Qatadah said, "They had no excuse.''

وكﻼ تبرنا تتبيرا

and each (of them) We brought to utter ruin. means, `We destroyed them completely.' This is like the Ayah,

وكم أهلكنا من القرون من بعد نوح

And how many generations (Qurun) have We destroyed after Nuh! (17:17) "Generations'' (Qurun) here refers to nations among mankind. This is like the Ayah,

ثم أنشٲنا من بعدهم قرونا ءاخرين

Then, after them, We created other generations (Qurun). (23:42) Some defined a generation as being 120 years, or it was said that a generation was one hundred years, or eighty, or forty, etc. The most correct view is that a generation refers to nations who are one another's contemporaries, living at the same time. When they go and others succeed them, this is another generation, as it was recorded in the Two Sahihs:

خير القرون قرني ثم الذين يلونهم ثم الذين يلونهم

The best of generations is my generation, then the one that follows it, then the one that follows that