To study comfort temperature, scientist have usually took a large set of volunteers (1) and placed them in controlled room, with fixed temperature, humidity, ventilation, clothing and physical activity.
Based on the reported impression, they have come with some ranges.
The American Heritage Dictionary of the English Language identifies room temperature as around 20 to 22 °C (68 to 72 °F), and the Oxford English Dictionary claims that it is "conventionally taken as about 20 °C (68 °F)".
Owing to variations in humidity and likely clothing, recommendations for summer and winter may vary; a suggested typical range for summer is 23 to 25.5 °C (73 to 78 °F), with that for winter being 20 to 23.5 °C (68 to 74 °F), although by other considerations the maximum should be below 24 °C (75 °F) – and to avoid sick building syndrome, below 22 °C (72 °F).
The World Health Organisation's standard for comfortable warmth is 18 °C (64 °F) for normal, healthy adults who are appropriately dressed. For those with respiratory problems or allergies, they recommend no less than 16 °C (61 °F), and for the sick, disabled, very old or very young, a minimum of 20 °C (68 °F).
(1) half seriously my professor at university used to say that usually these volunteers where students or soldiers, both cheap and available in large numbers.