Temperature is the physical property of a
system which underlines the common notion
of "hot" and "cold".
Two systems placed in contact come
ultimately into a state of mutual equilibrium
and said to be at the same temperature.
If a thermometer is placed in a gas container
then the thermal equilibrium is reached when
the energy exchange between the gas
molecules and the walls of thermometer
The energy exchange between gas and
thermometer is through collisions of gas
molecules with the walls of thermometers,