Wu Zetian (624 – 16 December 705), personal name Wu Zhao, was the empress regnant and only ruler of the (Wu) Zhou dynasty from 16 October 690 to 21 February 705. She had previously held power as the second wife and empress consort of Emperor Gaozong of the Tang dynasty and as empress dowager during the reigns of her sons Emperors Zhongzong and Ruizong of Tang, holding de facto power during these periods starting in 660. She was the only female sovereign in the history of China who is widely regarded as legitimate. During her 45 year reign, China grew larger, its culture and economy were revitalized, and corruption in the court was reduced. She was eventually removed from power during the Shenlong Coup [zh] and died a few months later.

In early life, Empress Wu was a concubine of Emperor Taizong. After his death, she married his ninth son and successor, Emperor Gaozong, officially becoming Gaozong's empress, the highest-ranking of his consorts, in 655. Empress Wu held considerable political power even before becoming empress, and began to control the court after her appointment. After Gaozong's debilitating stroke in 660, she became administrator of the court, a position with similar authority to the emperor's, until 683. History records that she "was at the helm of the country for long years, her power no different from that of the emperor". On Emperor Gaozong's death in 683, rather than entering retirement, and not interfering in the government, Empress Wu broke with tradition and took acquisition of complete power, refusing to allow either of her sons to rule. She took the throne in 690 by officially changing the name of the dynasty from Tang to Zhou, changing the name of the imperial family from Li to Wu, and holding a formal ceremony to crown herself as emperor.

Empress Wu is considered one of the great sovereigns in Chinese history due to her strong leadership and effective governance, which made China one of the world's most powerful nations. The importance to history of her tenure includes the major expansion of the Chinese empire, extending it far beyond its previous territorial limits, deep into Central Asia, and engaging in a series of wars on the Korean Peninsula, first allying with Silla against Goguryeo, and then against Silla over the occupation of former Goguryeo territory. Within China, besides the more direct consequences of her struggle to gain and maintain power, her leadership resulted in important effects regarding social class in Chinese society and in relation to state support for Taoism, Buddhism, Confucianism, education and literature.

Empress Wu played a key role in reforming the imperial examination system and encouraging capable officials to work in governance to maintain a peaceful and well-governed state. Effectively, these reforms improved the nation's bureaucracy by ensuring that competence, rather than family connections, became a key feature of the civil service. She also had an important impact upon the statuary of the Longmen Grottoes and the "Wordless Stele" at the Qian Mausoleum, as well as the construction of some major buildings and bronze castings that no longer survive. Besides her career as a political leader, Empress Wu also had an active family life. She was a mother of four sons, three of whom carried the title of emperor, although one held that title only as a posthumous honor. One of her grandsons became the controversial Emperor Xuanzong, whose reign marked the turning point of the Tang dynasty into sharp decline.

In Chinese history and literature, Wu Zetian (Mandarin pronunciation: [ù tsɤ̌ tʰjɛ́n]) was known by various names and titles. Mention of her in the English language has only increased their number. A difficulty in English translations is that they tend to specify gender (as in the case of "emperor" versus "empress" or "prince" versus "princess"), whereas, in Classical Chinese, words such as hou (后, "sovereign", "prince", "queen") or huangdi (皇帝, "imperial supreme ruler", "royal deity") are of grammatically indeterminate gender.

In Wu's time, women's birth names were rarely recorded. She changed her name to Wu Zhao after rising to power, often written as 武曌 (曌 has also been written as 瞾 on occasion, and both are derivatives of 照, which may be her original name), with 瞾 being one of the characters invented by Wu. Wu was her patronymic surname, which she retained, according to traditional Chinese practice, after marriage to Gaozong, of the Li family. Emperor Taizong gave her the art name Wu Mei (武媚), meaning "glamorous". Thus, Chinese people often refer to her as Wu Mei or Wu Meiniang (武媚娘) when they write about her youth, as Wu Hou (武后) when referring to her as empress consort and empress dowager, and as Wu Zetian (武則天) as her posthumous title, given only after her death.

Because her family name, Wu (武), is a homophone for the second character in 'parrot' (鹦鹉), there are many stories and jokes that make use of imagery of a parrot to communicate about Wu and her clan. In particular, Emperor Gaozong's family name, Li (李), is a homophone with a type of cat – so a story circulated about a cat eating a parrot at court.

During her life, and posthumously, Wu was awarded various official titles. Both hou (后) and huangdi (皇帝) are titles (modifications, or added characters to hou are of lesser importance). Born Wu Zhao, she is not properly known as "Wu Hou" (Empress Wu) until receiving this title in 655, nor is she properly known as "Wu Zetian", her regnal name, until 690, when she took the title huangdi.