Thermal Equilibrium
Definition
When two objects in contact reach the same temperature and heat stops flowing between them.
Technical definition
The state of zero net heat flow between systems in thermal contact, characterized by uniform temperature. The zeroth law of thermodynamics states that equilibrium is transitive — the basis of all thermometry.
Example
Leave a thermometer in your mouth for a minute: it starts at room temperature, heat flows from you into it, and when the reading stops climbing, thermometer and body are in thermal equilibrium — that is the moment the reading is trustworthy.
Thermal equilibrium is not 'everything is cold.' A pizza fresh from a 250°C oven and the 250°C oven air are in thermal equilibrium — both blazing hot. Equilibrium only means temperatures match and net heat flow is zero; it says nothing about what that temperature is. Two ice cubes in a freezer are in equilibrium too.