cleft sentence

Could you explain the meaning of “cleft” in “cleft sentence”?
I consulted it in many dictionaries but not sure.
Is the sentence below a cleft sentence? I think it is because there is the structure “it is …that…”

It was estimated that the leisure revolution would take place by the turn of the last century with hours devoted to work falling to 25-30 per week.

According to that definition, I think yours is not a cleft sentence.

I agree that the sentence is not a cleft sentence. A cleft sentence is one in which the usual word order of three parts - Subject+Verb+Object is as it were interrupted and sometimes these are put in a different order so that one of the three can be highlighted. Take a traditional sentence: The woman opened the window . Now you could highlight ‘woman’ and say It was the woman who opened the window or you could highlight ‘window’ and say: It was the window that the woman opened

Thanks a lot, OTS and Alan.