A trend was a shift in societal opinions which led to action.
In 2369, Data attributed the dropping of the prefix "Pel" from Orton's family's surname to the recent Arkarian trend toward a society that was more egalitarian. (TNG: "Starship Mine")
In 2370, Garak advised Quark that there was nothing worse than following the wrong trend, and suggested that he discourage Natima Lang from associating with the Cardassian Underground, as she might get caught in the crossfire were they to become unpopular. (DS9: "Profit and Loss")