The part of physics that studies heat: how heat moves, how it turns into other kinds of energy (like movement or electricity), and how other kinds of energy turn into heat. It is the science behind engines, fridges, air conditioners, and power plants — anything that uses heat to do work, or uses work to move heat. People often meet it through its famous rules, called the laws of thermodynamics. One of them says energy never disappears; another says heat always moves from the hot thing to the cold thing by itself, never the other way round unless a machine pushes it. Note the -s at the end: it looks plural, but it is a subject name like 'physics' or 'economics', so we say 'thermodynamics is difficult', not 'are'. It is a technical word from science and engineering; you will see it in textbooks and hear it in classrooms, not in daily chat. Outside the lab, people sometimes wave at it as a joke for 'a rule nobody can break' — 'you can't beat thermodynamics' means the situation simply cannot work.