Two systems are said to be in thermal equilibrium if and only if:

Thermal equilibrium is a fundamental state in thermodynamics where two or more systems have reached the same temperature, resulting in no net flow of heat between them [1]. According to the Zeroth Law of Thermodynamics, if two systems are in thermal equilibrium, they exist at the same temperature [2]. Heat transfer naturally occurs from a warmer body to a cooler body when they are in contact [3]. This process continues until the temperature gradient is eliminated and both bodies attain the same temperature [4]. Once this state is achieved, the net heat transfer becomes zero [5]. Therefore, the defining characteristic of two systems in thermal equilibrium is that there is no heat flow between them, even if they are connected by a path permeable to heat [5].

Sources

1. [1] https://satheejee.iitk.ac.in/article/chemistry/chemistry-thermal-equilibrium/
2. [3] https://www.grc.nasa.gov/www/BGH/heat.html
3. [4] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 8: Solar Radiation, Heat Balance and Temperature > HEATING AND COOLING OF ATMOSPHERE > p. 68
4. [5] https://en.wikipedia.org/wiki/Thermal_equilibrium