Fashion: Something that is widely done - worn, said, believed, pretended - by many members of some society at some time, usually for no better or other reason than that many do it.
There are fashions in very many human things, from clothes to opinions, and since most men follow leaders and imitate others, this is and will always be so.
Three important things to note about fashions are that
(1) fashions rule the majority in every field of human endeavor, also in such reputedly rational fields as science and philosophy;
(2) wherever there are fashions, there always is one dominant; and
(3) although fashions are not rational but imitative, they are quite strong motivators: Most people like to be, feel, look and behave like most of their mates.
Part of the explanation that fashions rule the majority is that most men are followers, and tend to be totalitarian.