There is no definition for it. The word itself merely means someone who adheres to tradition as opposed to modernization. Thats it in a nutshell.
In the Church the term is fraught with division and cannot even begin to explain or define those that call themselves traditionalists.
It can describe someone who is a raving sedevacantist or someone who prefers communion the hand as being a more ancient practice. It can describe the members of the SSPX or conversely the FSSP. It can and often is used as a perjorative to belittle those not comfortable with the changes that took place after Vatican II… It is also used as a label by those who feel they are following the true path. It is a complex issue and not easily defined.
I would offer the following. In the aftermath of Vatican II there was a general rush to strip Catholicism of everything and everything that was distinctly Catholic in nature. That included changes in the Mass and Liturgy to make it more accomodating and allegedly more relevent to the people and to other faiths, Churches were modernized,

altars moved and changed, and religious art moved removed or in some cases destroyed. There was a change in the emphasis on personal sin. Sin became thought of as more societal than personal in nature and as a result the use of confession by the faithful plummeted. It just wasn’t your fault anymore.
Catechisis suffered as the old Baltimore Catechism used for many years was suddenly no longer good enough. But there was nothing to replace it, so anyone with any inclination to do so could publish their own catechisms and many did. There were some real beauties floating around.
Devotions practiced for years, Novenas, Rosaries, 40 Hours, First Fridays, First Saturdays, Expositions Benedictions, Corpus Christi Processions etc pretty much disappeared. You just didn’t need these things anymore. They were unnecessary and maybe even harmful. Devotion to the Blessed Mother in particular took a nose dive and has really only recently started to make a comeback.
Latin for years described as a unifying force within the Church was removed from the Mass and replaced with the vernacular’ But Latin was kept as the official language of the Church and was supposed to retain a place of prominence within the Church
The Mass itself came to be thought of as more of a communal meal than the Eternal Sacrifice and was treated accordingly. Since it was no longer primarily the re-presentation of Christs death at Calvary, why pay any particular reverence when attending?. It was primarily a social gathering and peoples attitudes reflected that understanding.
For some reason, not really well explained by anyone, Holy Communion came to be distributed with the faithful standing and received in the hand, rather than on the tongue.The standing allegedly was done to imitate the Eastern Churches manner of reception. Receiving in the hand has never been adequitely explained.
So I would say a Traditionalist is one who doesn’t agree with the above changes, none of which were mandated by Vatican II by the way and laments the passing of those traditions which clearly showed the world that we were Catholics and helped to solidify our identity. Traditionalists feel something unique has been lost and needs to be recovered.
I attended a Corpus Christi procession last year in New Orleans. It was very nice, slightly over a mile in length and went from the Immaculate Conception Church to the St Louis Cathedral and was led by the Archbishop. There were probably over 500 people in the procession. While walking I struck up a conversation with an African American woman who was walking also. She told me that she was so happy that there were Corpus Christ processions again and remarked that as a small girl she really enjoyed seeing them and participating in them. Then she floored me. She said that even though she wasn’t Catholic, she loved the way that Catholics had never been afraid to show what they were, were not afraid to proclaim their faith and belief in what the Church taught and were not ashamed to show it in such a fashion as the processions. She said that she was so glad that Catholics seemed to be re-discovering what they were and what made the Catholic faith so unique…
I have never heard it stated any better.
