The western image of an immortal is someone who lives forever. One of the main terms used to describe an immortal in Chinese is Xiān 仙, and becoming such a being has been one of the primary goals of Daoism. The character Xiān 仙 has two parts, the first meaning person and the second meaning mountain. Thus an immortal is a person who lives on a mountain.
Mountains are holy places in Daoism. One of the reasons for this is that it is very hard to build on mountains, and thus they remain bastions of an archaic heritage that pre-dates the negative effects of civilisation. Daoist asceticism has been practiced on mountains four thousands of years, pre-dating the historical development of the religion itself.
A Daoist immortal is one who has come to the realisation that they are not separate form Nature, but wholly part of it. This realisation is not of the mind alone, but of the mind, body and spirit acting together in unity. Thus, one who is not differentiated from Nature is immortal in the same way that Nature is immortal.