Dic: the one (as the only person or thing)

2 [ADJ] det ADJ △If you say that someone or something is the one person or thing of a particular kind, you are emphasizing that they are the only person or thing of that kind.(= only)

• They had alienated the one man who knew the business...
• His one regret is that he has never learned a language.

CF.

3 [DET] DET sing-n △One can be used instead of `a' to emphasize the following noun.

• There is one thing I would like to know-What is it about Tim that you find so irresistible?...
• One person I hate is Russ.

4 [DET] DET adj sing-n △You can use one instead of `a' to emphasize the following adjective or expression. (INFORMAL)

• If we ever get married we'll have one terrific wedding...
• It's like one enormous street carnival here.

.... COBUILD