The ḥametz that the Torah forbade is ḥametz gamur (absolute ḥametz), meaning that the leavening process has been completed, and the food has become edible. But if fermentation had begun but not concluded, and if the food is edible under pressing circumstances, then it is “ḥametz nuksheh.” According to most poskim, ḥametz nuksheh is not forbidden by the Torah, but the Sages prohibited it so that people would not err and come to eat or keep real ḥametz.
An example of ḥametz nuksheh is the glue that scribes used to prepare from flour and water for gluing paper. Since its leavening process was never completed, and it is barely edible, it is ḥametz nuksheh, and the Sages forbade eating it or keeping it on Pesaḥ (MB 442:2). If its form was changed, as when the glue is used to stick papers together, then one is allowed to keep it. Others are more stringent and maintain that if the glue protrudes from between the pages, then it is considered as if it has maintained its same form, and it is forbidden to keep it on Pesaḥ (SA and Rema 242:3).
Similarly, dough that began to ferment to the point that its surface blanched, but the surface was not cracked as with true leavening, is ḥametz nuksheh, and it is forbidden by rabbinic law to eat or keep it on Pesaḥ (SA 459:2).