The Chinese sovereign was the ruler of a particular monarchical regime in the historical periods of ancient China and imperial China. Sovereigns ruling the same regime, and descended from the same paternal line, constituted a dynasty. Several titles and naming schemes have been used throughout Chinese history.
Sovereign titles
Emperor
The characters Huang (皇 huáng "august (ruler)") and Di (帝 dì "divine ruler") had been used separately and never consecutively (see Three August Ones and Five Emperors). The character was reserved for mythological rulers until the first emperor of Qin (Qin Shi Huang), who created a new title Huangdi (皇帝 in pinyin: huáng dì) for himself in 221 BCE, which is commonly translated as Emperor in English. This title continued in use until the fall of the Qing dynasty in 1912.
From the Han dynasty, the title Huangdi could also be abbreviated to huang or di. The former nobility titles Qing (卿), Daifu (大夫) and Shi (仕) became synonyms for court officials.
The power of the emperor varied between emperors and dynasties, with some emperors being absolute rulers and others being figureheads with actual power lying in the hands of court factions, eunuchs, the bureaucracy or noble families. In principle, the title of emperor was transmitted from father to son via primogeniture, as endorsed by Confucianism. However, there are many exceptions to this rule. For example, because the Emperor usually had many concubines, the first born of the empress (i.e. the chief consort) is usually the heir apparent. However, Emperors could elevate another more favoured child or the child of a favourite concubine to the status of Crown Prince. Disputes over succession occurred regularly and have led to a number of civil wars. In the Qing dynasty, primogeniture was abandoned altogether, with the designated heir kept secret until after the Emperor's death.
Of the San Huang Wu Di, the three first of them were called 皇 (huang, "august (ruler)") and the five last were called 帝 (di, "divine ruler"), which can translate as either emperor, demigod, divine ancestor, or superhuman. This title may have been used in the Shang and Xia dynasties, though oracle bones were found from the Shang dynasty showing the title 王 (wáng, "king").
King
The king (王, wáng) was the Chinese head of state during the Zhou dynasty. Its use during the Xia and Shang is uncertain but possible: the character has been found upon oracle bones. It was abolished under the Qin and, after that, the same term was used for (and translated as) royal princes. The title was commonly given to members of the Emperor's family and could be inherited. A poem from about 2,500 years ago said "普天之下,莫非王土.率土之賓,莫非王臣" which roughly translates as "Under the sky, nothing isn't the king's land; the people who lead the lands, no one isn't the king's subjects."
Son of Heaven
The Son of Heaven was a title of the Emperor based on the Mandate of Heaven. The Son of Heaven is a universal emperor who rules tianxia comprising "all under heaven". The title was not interpreted literally. The monarch is a mortal chosen by Heaven, not its actual descendant. The title comes from the Mandate of Heaven, created by the monarchs of the Zhou dynasty to justify deposing the Shang dynasty. They declared that Heaven had revoked the mandate from the Shang and given it to the Zhou in retaliation for their corruption and misrule. Heaven bestowed the mandate to whoever was best fit to rule. The title held the emperor responsible for the prosperity and security of his people through the threat of losing the mandate.