the (in) thing
The newest, most recent, or most current trend, fashion, or style. Deep V-necks are becoming the in thing for wedding dresses lately. Smooth, edgeless bodies are the thing for smartphone designs these days.
the thing
That which is currently (and usually temporarily) fashionable or trendy. Haven't you heard? Wearing knee-high socks is the thing now!
the thing
Also, the in thing. Something in style or vogue, the latest trend. For example, Wearing oversized clothes is the thing these days, or Vans and sports utility vehicles have been the in thing for some time now. The first term dates from the mid-1700s; the second, which uses in in the sense of "popular," dates from the mid-1900s. Also see the latest.