according to a long-standing usage a dogma is now understood to be a truth appertaining to faith or morals, revealed by God, transmitted from the Apostles in the Scriptures or by tradition, and proposed by the Church for the acceptance of the faithful. It might be described briefly as a revealed truth defined by the Church -- but private revelations do not constitute dogmas, and some theologians confine the word defined to doctrines solemnly defined by the pope or by a general council, while a revealed truth becomes a dogma even when proposed by the Church through her ordinary magisterium or teaching office. A dogma therefore implies a twofold relation: to Divine revelation and to the authoritative teaching of the Church.Another word, doctrine, is sometimes used to differentiate between "truth appertaining to faith or morals, revealed by God and taught by the Church", and anything else the Church teaches, even if not strictly speaking a dogma. For example, the late John Paul II advanced a certain view on human sexuality, so-called Theology of the Body. It is something the Chruch considers true and helpful, but a good Catohlic may still disagree with it, or over time it may be reviewed and changed.
A discipline is something pertaining to ceremony and behavior; it does not teach anything in itself. It comes from the authority of the Church, but is not claimed to be a revealed truth. It can change, but perhaps not for simply practical reasons, -- there has to be a connection to the formation of the faith. This is why I hesitate to agree that priestly celibacy can change merely to get more priests; for one thing, celibacy not only forms an obstacle for some, but it also attracts those who seek a more complete transformation of self. Celibacy connects to the theological fact that a priest is married to the Church. It is not likely to change as a matter of convenience.
Here is a good overview of these three terms: Dogma, doctrine, and discipline
Thanks very much for your comments, and for the great link to the differences between dogma, doctrine, and discipline. I found it interesting that the author said that priestly celibacy and the ordination of women are matters of discipline, the most easily changed. As an outsider, I have to say that from almost all of the Catholics I have spoken with, none of them thinks either of these will ever be changed, especially having women priests.
Unfortunately, Protestants are all over the place on the issue of women clergy. To my knowledge, Southern Baptists have not crossed that line yet, and have no plans to do so.