Title: Prime minister
Content:  The term prime minister in its French form, premier ministre, is attested in 17th Century sources referring to Cardinal Richelieu after he was named to head the royal council in 1624. The title was however informal and used alongside the equally informal principal ministre d'État ("chief minister of the state") more as a job description. After 1661, Louis XIV and his descendants refused to allow one of their ministers to be more important than the others, so the term was not in use. The term prime minister in the current sense originated in the 18th century in the United Kingdom when members of parliament disparagingly used the title in reference to Sir Robert Walpole (whose official title was First Lord of the Treasury). During the whole of the 18th Century, Britain was involved in a prolonged conflict with France, periodically bursting into all-out war, and Britons took outspoken pride in their "Liberty" as contrasted to the "Tyranny" of French Absolute Monarchy; therefore, being implicitly compared with Richelieu was no compliment to Walpole. Over time, however, the title became honorific and remains so in the 21st century.
Title: List of prime ministers of the United Kingdom
Content:  The prime minister of the United Kingdom is the principal minister of the crown of the Her Majesty's Government, and the head of the British Cabinet. There is no specific date for when the office of prime minister first appeared, as the role was not created but rather evolved over a period of time through a merger of duties. However, the term was regularly, if informally, used of Walpole by the 1730s. It was used in the House of Commons as early as 1805, and it was certainly in parliamentary use by the 1880s. In 1905, the post of prime minister was officially given recognition in the order of precedence. Modern historians generally consider Sir Robert Walpole, who led the government of Great Britain for over 
Title: History of the prime minister of the United Kingdom
Content:  The first official recognition given to the office had only been in the Treaty of Berlin in 1878, when Disraeli signed as "First Lord of the Treasury and Prime Minister of her Britannic Majesty". Not until seven years later, in 1885, did the official records entrench the institution of prime minister, using "Prime Minister" in the list of government ministers printed in Hansard. Incumbents had no statutory authority in their own right. As late as 1904, Arthur Balfour explained the status of his office in a speech at Haddington: "The Prime Minister has no salary as Prime Minister. He has no 
Title: First Lord of the Treasury
Content:  As of the beginning of the 17th century, the running of the Treasury was frequently entrusted to a commission, rather than to a single individual. Since 1714, it has permanently been in commission. The commissioners have always since that date been referred to as Lords Commissioners of the Treasury, and adopted ordinal numbers to describe their seniority. Eventually in the middle of the same century, the first lord of the Treasury came to be seen as the natural head of the overall ministry running the country, and, as of the time of Robert Walpole (Whig), began to be known, unofficially, as the prime minister. The term prime minister was initially, but decreasingly, used as a term of derogation; it was first used officially in a royal warrant only in 1905. William Pitt the Younger said 
Title: Cultural depictions of prime ministers of the United Kingdom
Content:  Cultural depictions of prime ministers of the United Kingdom have become commonplace since the term's first use in 1905. However, they have been applied to prime ministers who were in office before the first use of the term. They are listed here chronologically from the date of first appointment as prime minister.
Title: List of parliaments of the United Kingdom
Content: The term Prime Minister was not officially recognised until Campbell-Bannerman, although the title had been in common use, often as an insult, since 1721. 
Title: Prime minister
Content:  illness of George III, consolidated the power of the post. The title was first referred to on government documents during the administration of Benjamin Disraeli but did not appear in the formal British Order of precedence until 1905. The prestige of British institutions in the 19th century and the growth of the British Empire saw the British model of cabinet government, headed by a prime minister, widely copied, both in other European countries and in British colonial territories as they developed self-government. In some places alternative titles such as "premier", "chief minister", "first minister of state", "president of the council" or "chancellor" were adopted, but the essentials of the office were the same.
Title: History of the prime minister of the United Kingdom
Content:  of the position of prime minister and the title was more commonly used, if only unofficially. Associated initially with the Whigs, the Tories started to accept it. Lord North, for example, who had said the office was "unknown to the constitution", reversed himself in 1783 when he said, "In this country some one man or some body of men like a Cabinet should govern the whole and direct every measure." In 1803, William Pitt the Younger, also a Tory, suggested to a friend that "this person generally called the first minister" was an absolute necessity for a government to function, and expressed his 
Title: Prime minister
Content:  makes no mention of a prime minister. Though it had de facto existed for centuries, its first mention in official state documents did not occur until the first decade of the twentieth century. Accordingly, it is often said "not to exist"; indeed there are several instances of parliament declaring this to be the case. The prime minister sits in the cabinet solely by virtue of occupying another office, either First Lord of the Treasury (office in commission) or more rarely Chancellor of the Exchequer (the last of whom was Balfour in 1905). During the period between the time it is clear that the incumbent government has been defeated at a general 
Title: History of the prime minister of the United Kingdom
Content:  in the Commons; and, the lord treasurer assumed a leading position among ministers. The power of financial initiative was not, however, absolute. Only ministers might initiate money bills, but Parliament now reviewed and consented to them. Standing Order 66 therefore represents the beginnings of Ministerial responsibility and accountability. The term "Prime Minister" appears at this time as an unofficial title for the leader of the government, usually the head of the Treasury. Jonathan Swift, for example, wrote that in 1713 there had been "those who are now commonly called Prime Minister among us", referring to Sidney Godolphin, 1st Earl of Godolphin and Robert Harley, Queen Anne's Lord Treasurers and chief ministers. Since 1721, every head of the Sovereign's government – with one exception in the 18th century (William Pitt the Elder) and one in the 19th (Lord Salisbury) – has been First Lord of the Treasury.

Question: when was the term prime minister first used?
Answer: 1624.
18th century.