If you are looking for that which comes at the end of a sentence, you are in the wrong place.
For the interior design profession, period means the time space in history in which a design was manifest.
Ideally, the designs of a period should represent the materials, techniques, life styles, arts, economics, government, politics, discoveries, religions, et cetera that express the best essences of it’s time space in history.
It may also include fads, fashions, popularity, et cetera which are not of the highest level of intelligent aesthetics.
It is usually the most intelligent and highest quality segments of past periods that last.
With intelligence, imagination, creativity, and knowledge of the elements and characteristics of aesthetics and how to relate each to the other in an organized and balanced manner, different periods of interior design can be used in the same composition.
Interior design, music, science, government, economics, life styles, medicine, the arts, religions, and all other aspects of human existence and civilization are all parts of what we know as history.
Is it of merit for you to know and be aware of this history? How could it affect your day-to-day existence? And your aesthetic experiences?
Food for thought.