What is the name of the boundary between the Sun's chromosphere and corona?

The boundary-like layer between the Sun’s chromosphere and corona is called the transition region.

The solar transition region is not a sharply defined solid surface or fixed-altitude boundary. It is a thin, highly variable part of the Sun’s atmosphere where temperature, ionization, radiation, and the dominant physical forces change rapidly between the cooler chromosphere and the much hotter corona.

Temperatures rise dramatically through this region, reaching roughly 100,000 kelvins in material associated with the transition. Above it, the corona can reach around one million kelvins or more. This sudden increase is related to changes in how atoms such as helium radiate energy and become ionized, although the broader problem of coronal heating remains an active area of research.

The transition region is also where the character of the atmosphere shifts. Gas pressure and fluid motion are especially important below, while magnetic forces increasingly shape structures above. “Photosphere” is a common wrong answer because it is the visible surface beneath the chromosphere. “Heliopause” is much farther away, marking the outer boundary of the Sun’s solar-wind bubble.