What Hi-Fi is a website that provides reviews and recommendations for headphones and other consumer technology products. They have a list of the best headphones in 2023, which includes the Sony WH-1000XM5, Sony WF-C500, Sony WH-CH720N, Sony WF-1000XM5, and Apple AirPods Pro 2. They also have articles that discuss the differences between closed-back and open-back headphones, and the best headphones for different types of buyers.

Based on the search results, there is no specific definition of "hi-fi headphones." However, the term "hi-fi" generally refers to a setup or device that produces a high-fidelity audio signal with low distortion, an accurate frequency response, and good technical qualities. Some of the best audiophile headphones in 2023 include the Sennheiser HD 800 S, HiFiMan Edition XS, and HiFiMan Sundara 2020. These headphones are generally more expensive and require a powerful amplifier to drive them.

In summary, "hi-fi headphones" are not a specific type of headphones, but rather refer to headphones that produce a high-fidelity audio signal. What Hi-Fi provides recommendations for the best headphones in 2023, while other sources such as RTINGS.com and PCMag provide reviews and recommendations for the best audiophile headphones.