A style or design that continues to be popular over an extended period of time, even though fashions change.

Answer: Classic

Explanation:

In Fashion Merchandise, a classic style or design is the one that remains popular over an extended time, virtually unaffected by the changes in fashion. Fashion is normally evolutionary, meaning that it continuously changes throughout the different seasons, or from one year to the next. On the other hand, a fad would be a temporary style, which attracts many people for a short period.