Catechism of the Catholic Church §2102 — excerpt
"A vow is a deliberate and free promise made to God concerning a possible and better good which must be fulfilled by reason of the virtue of religion," A vow is an act of devotion in which the Christian dedicates himself to God or promises him some good work.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: CIC, can. 1191 # 1