What is the definition of thermal equilibrium

Two physical systems are in thermal equilibrium if there is no net flow of heat between them.

Explanation:

Two Physical systems are said to be in thermal equilibrium if there is no net flow of heat energy between them when they are connected and both of the system is at the same temperature.

Thermal equilibrium is of two varieties i.e Two thermally connected bodies and Internal thermal equilibrium.

If two bodies are thermally connected to each other then there will only transfer of the heat to maintain equilibrium but no transfer of the matter and mass.

Also, a internal thermal equilibrium is maintained in a isolated body where there is no transfer of the heat from the body outside or inside of it.

Thermal equilibrium follows zeroth law of thermodynamics which says that Two bodies are in thermal equilibrium with each other if they individually are in thermal equilibrium with a third body.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.