Catechism of the Catholic Church §928 — excerpt
"A secular institute is an institute of consecrated life in which the Christian faithful living in the world strive for the perfection of charity and work for the sanctification of the world especially from within."
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. 710