Thermodynamics has an intricate etymology.
By a surface-level analysis, the word consists of two parts that can be traced back to Ancient Greek. Firstly,  ("of heat"; used in words such as thermometer) can be traced back to the root θέρμη therme, meaning "heat". Secondly, the word  ("science of force [or power]") can be traced back to the root δύναμις dynamis, meaning "power".
In 1849, the adjective thermo-dynamic is used by William Thomson.
In 1854, the noun thermo-dynamics is used by Thomson and William Rankine to represent the science of generalized heat engines.
Pierre Perrot claims that the term thermodynamics was coined by James Joule in 1858 to designate the science of relations between heat and power, however, Joule never used that term, but used instead the term perfect thermo-dynamic engine in reference to Thomson's 1849 phraseology.