Eutectic comes from the Greek eu tekein, to melt well. The British physicist Frederick Guthrie coined it in his paper On Eutexia, read to the Physical Society in London in May 1884 and printed in the Philosophical Magazine. His definition was plain: a body of two or more constituents in the proportion that gives a lower temperature of liquefaction than any other proportion.
Salt water and alloys
Guthrie started from salt water. Water freezes at 0 °C, yet a solution with salt at the eutectic ratio stays liquid down to about -21 °C and then solidifies as a whole at that temperature. Other ratios soften and melt across a range rather than at a single point. Guthrie went on to show the same behaviour in alloys of metals and of salts.
In the cold chain it is this fixed point that does the work: the fill inside a eutectic plate absorbs heat as it melts and stays close to its own melting point throughout the phase change. That temperature belongs to the fill; the load temperature also depends on insulation, the number of plates and how they are placed.
