Art historian Giorgio Vasari (1511-1574) used the term in his book the Lives in 1550.

He described a change in art and understood that something new and different was happening in the artworld.

They didn't just know it, they defined the renaissance as a golden age and in many ways the reason we speak of the Renaissance as a thing is precisely because its protagonists very sucessfully argued for it as a new golden age of the arts. Let us consider the following quote from Leon Battista Alberti, speaking after returning from exile in 1434:

I used to marvel and at the same time to grieve that so many excellent and superior arts and sciences from our most vigorous antique past could now seem lacking and almost wholly lost. We know from [remaining] works and through references to them that they were once widespread. Painters, sculptors, architects, musicians, geometricians, rhetoricians, seers and similar noble and amazing intellects are very rarely found today and there are few to praise them. Thus I believed, as many said, that Nature, the mistress of things, had grown old and tired. She no longer produced either geniuses or giants which in her more youthful and more glorious days she had produced so marvelously and abundantly.

Since then, I have been brought back here [to Florence]--from the long exile in which we Alberti have grown old--into this our city, adorned above all others. I have come to understand that in many men, but especially in you, Filippo [Brunelleschi], and in our close friend Donato [Donatello]the sculptor and in others like Nencio [Ghiberti], Luca [Luca della Robbia]and Massaccio, there is a genius for [accomplishing] every praiseworthy thing. For this they should not be slighted in favour of anyone famous in antiquity in these arts. Therefore, I believe the power of acquiring wide fame in any art or science lies in our industry and diligence more than in the times or in the gifts of nature.

(Alberti, On Painting, trans. John R. Spencer)

What we see here is that Alberti is defending precisely such an idea, that out of seeming barrenness the arts had been revived. This of course leads to the very interesting question of how and why this idea was constructed and defined. Part of it was that the earliest Renaissance buildings were part of a general project of civic reconstruction as Florence sought to project itself and the various competing political factions sought to increase their power by sponsoring building projects in the public spaces of the city. A classical model here had much to recommend it, carrying overtones of great antiquity and solidity(as well as independence from foreign interference), and indeed partisans of the classical revival contrasted it with the imported and inappropriately "Frankish" Gothic style even as it was treated as the new and better style. Moreover, the popularity of classical education and study of latin and greek authors like Cicero made it easier and easier to define artistic ideals in precisely the terms found in latin authors like Vitruvius. To be sure, these arguments conceal just how diverse Renaissance art and architecture is and how durable the International Gothic style actually turned out to be but they are still arguments that define certain norms and what we expect to find when we look at Renaissance art.