Systems involving measurements of temperature or exchanges of thermal energy are thermal systems.
Thermal systems are characterized by their temperatures. Temperature is determined by the mean of the random motions of the atoms making up a material. A device that measures temperature is known as a thermometer. Temperatures are measured by assigning numbers to systems we designate as cold or hot and by using the idea of thermal equilibrium.
If a spoon from the drawer is put into a pot of boiling water, thermal energy will flow from hot water to cooler spoon. Thermal energy is exchanged between the water and the spoon, but eventually the spoon and water do not exchange any more energy. Thermal equilibrium is the state in which there is no net interchange of thermal energy between bodies. The spoon comes to thermal equilibrium with the boiling water.
Temperatures are measured by allowing the thermometer and the system to come to thermal equilibrium. Two bodies are said to be in a state of thermal equilibrium if there is no net interchange of thermal energy between those bodies when they are brought into thermal contact. The measurement of the temperature of the thermometer therefore also gives a measurement of the temperature of the system.
The most common temperature scale used on Earth, and often used in science, is the Celsius scale. It sets the zero temperature at the freezing point of water and sets 100 °C at the boiling point of water at sea level under standard conditions. The choice of the boiling end point is arbitrary scientifically. As virtually everyone living in the temperate regions of Earth knows from personal experience, there are temperatures below zero using this scale. A brisk day might have a temperature around 10°C, and a very hot day a temperature around 40°C. Human body temperature is 37°C. Comfortable room temperature is about 20°C.
In science, the arbitrariness of the Celsius scale makes it less useful in communication. Lord Kelvin devised a temperature scale, known as the absolute temperature or the Kelvin temperature scale, that begins with the lowest temperature possible in the universe as 0 K (absolute zero) and has a temperature of 273.15 K at the triple point of water (the temperature at which ice, water, and water vapor are in thermal equilibrium). With this definition, the kelvin interval is the same as the degree Celsius (1K = 1°C).