Trends vs. fads
Here is how Wikitrends differentiates the two:
Trends are differentiated from fads primarily by longevity. While fads may come and go as quickly as a “flash in a pan,” trends are longer lasting and often herald more permanent cultural change. While fads can be fun, only trends are stable and enduring enough to move society and build a business plan around.
Of course, in the heat of the moment it’s difficult to differentiate between trends and fads.
Before you take Wikitrends as gospel, however, consider that it also came up with the following statement:
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