a sentence in which a simpler sentence is paraphrased by being divided into two parts, each with its own verb, in order to emphasize certain information, esp. a sentence beginning with expletive it and a form of be followed by the information being emphasized, as It was a mushroom that Alice ate instead of Alice ate a mushroom

a two-part sentence in which a subject or subject complement expresses information being emphasized and an indefinite relative clause corresponds to the rest of a simpler sentence that has been paraphrased, as A mushroom was what Alice ate or What Alice ate was a mushroom