Ten times as much, or ten times as many. It works two ways in a sentence. As an adjective before a noun: a tenfold increase, a tenfold rise in prices. As an adverb after a verb: sales increased tenfold, the population grew tenfold. Both are common; the adverb use feels slightly more natural in speech. What makes it worth learning is that it is a whole family, not one word — twofold, threefold, fivefold, a hundredfold all work the same way, and once you see the pattern you own all of them. The word is a bit formal, and it lives where numbers live: reports, news, science writing, business talk. One thing English does here that Chinese does differently: tenfold means the new amount is ten times the old one, so 100 becoming 1,000. Careful writers use it for increases far more often than decreases — "decreased tenfold" is heard, but many readers find it awkward and prefer "fell to a tenth." People also stretch it to things you cannot count: "her confidence grew tenfold" does not mean anyone measured it; it means a lot. That looser use is normal in speech but a bit much in formal writing.