What is the definition of modern interior design?

Technically speaking, in interior design the Word "Modern" refers to a specific period in US design history that occurred during the mid-20th century. It's best recognized by clean lines, well-made, iconic furniture whose designs still have relevance today.

The Word "modern" is also sometimes also used synonymously with "contemporary" when people are describing what is current in the culture at the moment. For this reason many designers refer to the historical design style as "Mid-Century Modern" to distinguish it from other conversational descriptors.