We hear a lot of Catholics calling using official sounding Catholic words make their arguments or to impugn others, but do we all really understand what these terms mean?
I will try to explain, in the most basic terms possible, the meaning behind the modern uses of these words and how I think they might hit or miss the mark.
Sedevacantist. We hear this one a lot. The real definition is simple. Sedevacantists hold that, because modern (recent) Pope's have said or done things that seem to be in contradiction to the Tradition of the Church AND that Popes, by virtue of their infallibility, must be perfect in such matters, therefore these Popes must not really be Popes. Therefore, sedevacantists believe that the see of Peter is empty or sede vacante. As a group, sedevacantists are the smallest of subsets of the smallest of subsets. You probably have never met one and you should be glad for that fact.
Misuse: So if these misguided folks represent such small real numbers, why do we regularly hear the term? Simply, many people use the term to derisively describe Traditionalists in the Church that publicly hold to the first part of the formula I put forth above. If someone holds to the notion that Pope's have said or done things that seem to be in contradiction to the Tradition of the Church, well then they must be sedevantists. They want you to believe that such people are a fringe of a fringe and to be discounted. But they miss the other critical element necessary to the moniker, that these sedevacantists must also believe that the Pope must be perfect or he is not a Pope.
Most of those to whom the sedevacatist pejorative is applied reject the notion that the Pope must be perfect. So this discussion naturally leads to another term...
Ultramontanist. Generally, this term means someone who ascribes vast prerogatives and powers upon the office and person of the Pope. This term has a long history as an insult, even if its meaning has morphed considerably over the centuries. It was commonly used as an insult against Catholics by protestant sects.
This one is trickier because its definition is not truly fixed, but in modern parlance, it has come to describe a Catholic that takes the view that all that is needed to be Catholic is to agree with the Pope. One need only follow the Pope because the Pope is the infallibly perfect embodiment of what it means to be a Catholic today, a cult of the Papacy. Again, probably only a tiny subset of a subset actually hold to this position.
Misuse: The term is used to deride those who deplore criticism of the Pope. This insult is sometimes framed to make you believe that anyone who gives the benefit of the doubt to the Pope has his reason overwhelmed by the cult of the Papacy. More generally it is applied to those that minimize or deny the break with Tradition that many Catholics see as obvious, and or take the view that all Catholics should give general acceptance and deference to papal statements or actions and that criticism of same is to be avoided. This is an overly broad use of the term used to sweep in those who dislike criticism of the Pope with those who believe such criticism is un-Catholic.
In reality, sedevacantism and ultramontanism are two sides of the same coin. Both believe the Pope must be perfect. The sedevcantist excoriates the imperfection by denying the chair is occupied while the ultramontanist embraces the man and excoriates anyone who points out imperfection. In that, they both fail to be fully Catholic.
Sensus Fidelium. There is probably no term more misused and misunderstood than this one. Sensus Fidelium, the catechism teaches us, is "the supernatural appreciation of faith on the part of the whole people, when, from the bishops to the last of the faithful, they manifest a universal consent in matters of faith and morals." In short, it is the appreciation and consent of the faithful to that which has always been true.
Misuse: Today, many people take the view that the Sensus Fidelium is some sort of Catholic House of Commons populated by the people. A sort of supreme Catholic legislature unfettered by the strict construction of revelation, free to override all Catholic teaching up to and including the very words of Jesus. That somehow, if enough people are gathered in the name of falsehood, we collectively have the power to change God's mind as if He is some petty tyrant or bureaucrat that should rubber-stamp the contrary will of the people, for fear of them.
Contraception is the perfect example. Some refer to the widespread and common use of contraceptives by 'Catholics' and rejection of the Church's teaching on same, as the 'sensus fidelium.' That rejection of truth by a majority has the power to compel the Church (and God) to change the truth, to declare a new truth.
In reality the 'sensus fidelium' has the power to declare nothing, its role is to nod and kneel in joyful acceptance of the truth.
There are many more words that are generally misunderstood and misused, but they are for a future post.