Covers the study of (mostly homogeneous) macroscopic systems from a heat/energy/entropy point of view. Consider also using tag: [statistical-mechanics].
When to Use this Tag
thermodynamics should be used if your question discusses the time-evolution of a mostly homogeneous, macroscopic system – for example a cold bottle of water in a warm bathtub. You can combine this tag with statistical-mechanics for a microscopic derivation of thermodynamical laws, and maybe with the other ‘macroscopic’ tags such as cosmology or gravity, depending on your problem. Tagging a question as both thermodynamics and quantum-mechanics usually makes no sense, unless in the context of thermal-radiation.