From Glossary of Meteorology
The isotherm connecting places that have a mean temperature of 10°C (50°F) for the warmest month of the year.
It was adopted by Supan (1879) as the climatic isopleth that most nearly corresponded to the arctic tree line, and later by Köppen (1900) as indicating the division between tundra climate and the tree climates.
Supan, A. 1879. Die Temperaturzonen der Erde. Petermanns Geog. Mitt. 25. 349–358.
Köppen, W. 1900. Versuch einer Klassification der Klimate vorsugsweise nach ihren Bezichungen zur Pflanzenwelt. Geograph. Zeirsehr. 6. 593–611, 657–679.