from A Catholic Dictionary, edited by Donald Attwater, Second edition, revised 1957
Tradition: The sum of revealed doctrine which has not been committed to sacred Scripture (though it may have appeared in uninspired writing) but which has been handed down by a series of legitimate shepherds of the Church from age to age. As revelation is must have come to the Apostles directly from the lips of Christ or been handed down by the Apostles at the dictation of the Holy Ghost. More broadly the term is used for the sum of doctrine revealed either in Scripture or by word of mouth: so in 2 Thess. ii, 14: "Hold by the traditions you have learned, in word or in writing, from us."
from Modern Catholic Dictionary by John A. Hardon, S.J. Doubleday & Co., Inc. Garden City, NY 1980
Literally a "handing on," referring to the passing down of God's revealed word. As such it has two closely related but distinct meanings. Tradition first means all of divine revelation, from the dawn of human history to the end of the apostolic age, as passed on from one generation of believers to the next, and as preserved under divine guidance by the Church established by Christ. Sacred Tradition more technically also means, within this transmitted revelation, that part of God's revealed word which is not contained in Sacred Scripture. Referring specifically to how Christian tradition was handed on, the Second Vatican Council says: "It was done by the apostles who handed on, by the spoken word of their preaching, by the example they gave, by the institutions they established, what they themselves had received -- whether from the lips of Christ, from His way of life and His words, or whether they had learned it by the prompting of the Holy Spirit" (Constitution on Divine Revelation, II, 7). (Etym, Latin traditio, a giving over, delivery, surrender; a handing down: from tradere, to give up.
from The Catechism of the Catholic Church, second edition, English translation 1994
81 "Sacred Scripture is the speech of God as it is put down in writing under the breath of the Holy Spirit." (Dei Verbum 9)
"And [Holy] Tradition transmits in its entirety the Word of God which has been entrusted to the apostles by Christ the Lord and the Holy Spirit. It transmits it to the successors of the apostles so that, enlightened by the Spirit of truth, they may faithfully preserve, expound, and spread it aboard by their preaching.