A marriage proposal was a proposal to get married.
History
Clara Oswald declined a marriage propsal from Emperor Ludens Nimrod Kendrick Cord Longstaff XLI. (TV: Nightmare in Silver)
In an attempt to expose her as a Zygon, the Tenth Doctor made a marriage proposal to Queen Elizabeth I of England in 1562, which she accepted. Though he took her acceptance as as proof that she was a Zygon, he was corrected when the true Zygon, detected by his Machine That Goes "Ding", revealed itself, having assumed the form of the Queen's horse. As a result of his proposal, the Doctor and the Queen were married, (TV: The Day of the Doctor) which he recalled as a "mistake". (TV: The End of Time)