The following definition of “Modernism” is pretty useful; note how a lot of modern Catholic pop apologists, including one intellectual fraud who has tried to argue icon veneration represents a “u-turn” but it is okay due to appealing to the “development of doctrine,” etc:
Modernism.
Modernism teaches that truth changes from age to age, and that the Church must
therefore change as well, in order to be “relevant” to the modern secular
world. Modernist clergy gut traditional Catholic worship, doctrine and morality
by filtering it through modern relative philosophy and various secular “dogmas”
and “values.” Modernists strip the faith of those teachings and practices the
modern world deems intransigent, exclusivity, difficult, unenlightened,
fanatical or embarrassing. As a result, the notion of objective religious truth
disappears, religion is reduced to little more than emotions and symbols, and
the principles of morality (if any) becomes fuzzy. (Anthony Cekada, Welcome
to the Traditional Latin Mass [West Chester, Ohio: St. Gertrude the Great
Church, 2006], 15)