Definition of period piece : a work (as of literature, art, furniture, cinema, or music) whose special value lies in its evocation of a historical period.

Big budget period pieces are often dramas, frequently big epic ones, and take place at least thirty years ago - because any period of time greater than 25 years tends to shade into "when your parents were kids", which would obviously be pretty unrecognizable to you.

Though "Game of Thrones" is not technically a period piece — it's a fantasy series set on a different world — so many elements so closely resemble the late medieval period that we're willing to let that technicality slide.