Isothermal

vertical solidus line in the phase diagram of peritectic binary systems

(s. Peritectic) which gives the peritectic temperature.

For

example, the iron-carbon phase diagram shows three isothermal reactions,

one peritectic (HIB line), one eutectic (ECF line) and one eutectoid

reaction (PSK line).

Additional references:
Peritectic
Peritectoid
Eutectic
Eutectoid