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) Some oaths were made as blood pacts. (AUDIO: Requiem for the Rocket Men)

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)

According to Leela, the Doctor taught her that "Oaths taken in ignorance mean nothing." The same applied to oaths made under duress, or when given no other choice. Furthermore, she held that it was one's moral obligation to break an oath which was an "oath of evil". (AUDIO: Requiem for the Rocket Men)

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)