A vow of abstinence was an oath made by an individual to abstain from sex.
It was a religious practice among the Cult of the Pah-wraiths. They claimed it was in line with the practices of ancient Bajorans, who better understood the value of denying worldly pleasures. Cult members would take a vow of abstinence as part of their covenant with their master, Dukat. Exceptions were granted only when he believed husband and wife had demonstrated a true religious bond. (DS9: "Covenant")