Update : 26/02/2019
Modern physics is the post-Newtonian conception of physics. It implies that classical descriptions of phenomena are lacking, and that an accurate, "modern", description of nature requires theories to incorporate elements of quantum mechanics or Einsteinian relativity, or both. In general, the term is used to refer to any branch of physics either developed in the early 20th century and onwards, or branches greatly influenced by early 20th century physics.
EMRS, PHYSICS DEPT.