Vow

A vow was a promise made, a commitment made under oath to another individual, to a larger entity, like a deity or organisation, or indeed to oneself.

Leela believed that one should not make a vow they cannot keep. According to the Fourth Doctor, some monks took a vow of silence. (AUDIO: The Darkness of Glass)

Vows were also involved in a formal wedding ceremony. Pete Tyler stumbled on his vows when he got married to Jacqueline Tyler. (TV: Father's Day)

The Doctor's promise
According to the Eleventh Doctor, the name the Doctor chose was "like a promise you make." (TV: The Name of the Doctor) Specifically, the name of "the Doctor" represented a vow he made to himself: "Never cruel or cowardly. Never give up, never give in." (TV: The Day of the Doctor)

The War Doctor, by engaging in the Last Great Time War and committing terrible acts "in the name of peace and sanity", was considered to have broken this promise, (TV: The Name of the Doctor) and so believed he did not deserve to be called the Doctor. He allowed himself to reclaim his title, just at the end, when the Doctor found another way to end the war, saving Gallifrey. (TV: The Day of the Doctor)