Dào is the pinyinromanization of the Chinese character 道, representing a word usually rendered in English as Tao, and used as the root word for the English term Taoism, a native Chinese philosophy and religion that, along with its various offshoot sects and syncretisms with other traditions (Chan Buddhism, Neo-Confucianism), has influenced much of East Asia for thousands of years. More recently, it has gained worldwide recognition.
The concepts of Taoism were first widely studied in the West before the development of pinyin, when the older Wade-Giles transliteration system was in use. Consequently, the Wade-Giles spellings are still generally used in most English language editions of the Tao Te Ching and other major Taoist works, and thus most commonly used and recognized by native speakers of most Western languages. "Taoism" appeared first in English in 1836 (Oxford English Dictionary) as a translation of the Chinese term 道教 (Wade-Giles tao4 chiao4, pinyin dào jiào).
However, in academia and politics, there has been a strong trend towards using pinyin, which is recommended by the Chinese government and prefered by many Chinese speakers. While "Mao Tze Tung" may still have some currency, the names of contemporary Chinese politicians are always rendered in pinyin. Joseph Lau's translation of the Tao Te Ching (pub. Penguin) has been renamed "Daodejing" in its latest edition; similarly, Burton Watson's translation of Chuang Tzu (pub. Columbia Univ. Press) is now titled "Zhuangzi" instead. Both texts have abandoned Wades-Giles in favor of pinyin.
Due to fundamental differences between Chinese and English phonology, neither d nor t can be considered adequate representations for the consonant at the beginning of the word Dao/Tao. The Chinese pronunciation is voiceless (like t and not like d), but it is also unaspirated (without the puff of air which is normally a part of English t but which is never a part of English d). Thus, both transliterations are equally close (or far) from the Mandarin pronunciation of 道. Note, however, that in the vast majority of cases pinyin is phonetically more accurate than Wade-Giles.
Generally, in the case of words with Chinese origins that have long been assimilated into English, opinions differ. Some people think that existing words in English which come from Chinese words should be remodeled after the pinyin transliteration scheme. Others think that the older forms should be retained because those spellings have become English words in their own right--and hence are not Chinese anymore--while new borrowings should be written according to the official transliteration scheme.
By consensus, the English language edition of Wikipedia prefers the English spellings - that is, Taoism and Tao Te Ching, for consistency across articles.
