Why is latent heat called hidden energy?

Scientists in the past were not sure where heat was going during phase changes.

In the past scientists investigated how much heat was required to raise the temperature of substances (heat capacity). During these experiments they noted that heating objects (i.e. transferring heat energy to them) caused their temperature to rise. But when the substance changed phase its temperature stopped rising (this only happened during phase change). The problem was that heat energy was still being transferred to the substance during phase change and by gaining heat energy the scientists of the time believed the temperature should still increase.

So the substance was gaining energy but it was "hidden" from observers because the temperature was not rising. That is why they called the heat they transferred to the substance during phase changes "" (i.e. hidden heat).

We now know that increasing temperature is linked to increasing kinetic energy of the molecules and that during an ideal phase change there is no increase in kinetic energy of the molecules. During phase changes heat energy is absorbed/lost to break/form bonds, i.e. the molecules gain/lose potential energy.