Original Meaning of Dao
'Dao' is a concept which appeared very early on; however, initially it was not a philosophical notion.
Originally, 'Dao' Referred to the Way Tread by People
The ancient dictionary Explanations to the Chinese Characters ( 說文解字 Shuowen Jiezi ) defines Dao as the way or raod along which one walks. There are many kinds of roads, such as branching roads and roads going in all directions, but only roads leading in a single, definite direction can be called 'Dao'. This original meaning was then extended to include the notions of 'rule' or 'law'. For example, the movement of the heavens follows regular laws, and is thus called the 'Dao (Way) of Heaven' ( 天道 Tiandao ). During the Spring and Autumns era, there was the saying 'The Way of Heaven is bright, and as regular as the Sun and Moon'. This meant that the movement of the heavens was very clear and could be observed through the movement of the Sun and Moon. Thus the ancient notion of the Way of Heaven acquired the abstract significance of 'law'. The term 'Dao of Heaven' gradually became a rich concept, until it lost its original meaning and came to designate natural laws and principles.