Like many others, the word trend has evolved and changed in meaning over time. Some 400 years ago trend meant “to run or bend in a certain direction” and pertained to natural occurrences like rivers, coastlines and mountain ranges. Some 150 years later it was adopted as a term for the way anything bends or its general course or direction. Today, and since about 1950, a trend has come to pertain predominantly to popular culture, fashion, and technology and means “a prevailing new tendency.”
The current definition of a trend has even been dissected and analyzed mathematically in an attempt to better understand how it occurs. Take this description, for instance, from an episode of the television show Numb3ers (notorious for explaining complex mathematical concepts in a straightforward and entertaining way):