So changed, damaged, or hidden that you cannot tell what or who it is anymore. "The town was unrecognizable after the fire." "She had lost so much weight that she was almost unrecognizable." The word carries a small shock inside it: there is something you used to know well, and now the knowing fails. That is why it turns up so often with almost, virtually, and completely — writers want to mark how far the change went. Notice that the change can be good or bad. A ruined building is unrecognizable; so is a shabby house after a beautiful renovation, and so is a shy student who has become a confident speaker. The word only measures distance from the old version, not whether the new one is better. It also gets used for things that were never damaged at all, only disguised or badly copied — a voice unrecognizable on a bad phone line, a face unrecognizable behind a mask, a dish so badly cooked it is unrecognizable as the thing it claims to be. That last pattern, "unrecognizable as X," is worth remembering. One more thing: the -ize spelling is standard in American English and common in British too, but British writers and British dictionaries often use unrecognisable with an s. Both are correct. Taiwan textbooks mostly follow the American z.