Wiktionary
n. As distinguished from classical physics, all physics discoveries made since approximately 1900.
Wikipedia
The term modern physics refers to 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.
Small velocities and large distances is usually the realm of classical physics. Modern physics, however, often involves extreme conditions: quantum effects typically involve distances comparable to atoms (roughly 10 m), while relativistic effects typically involve velocities comparable to the speed of light (roughly 10 m/s). In general, quantum & relativist effects exist across all scales, although these effects can be very small in everyday life.
Usage examples of "modern physics".
For all they knew, the results would be inconsistent with General Relativity and they would have overturned one of the chief pillars of modern physics.
In the longer term it would lay the foundation for the whole of modern physics.
More than half a century later, his dream of a unified theory has become the Holy Grail of modern physics.
Such a screen is simply the one remote possibility which modern physics admits.
When it is published, it will shake the very foundation of modern physics.
Science and technology seemed to have given ver everything ve could ask for: an escape from the poisoned bat-tieground of gender, a political movement worth fighting for, and even a quasi-religion-insane enough in its own way, but unlike most other science-friendly faiths, at least it wasn't a laboriously contrived synthesis of modern physics and some dog-eared historical relic: a mock truce like the fatuities of Quantum Buddhism, or the Church of the Revised Standard Judaeo-Christian Big Bang.
It was this work, referred to as Two New Sciences, even more than his support for Copernicus, that was to be the genesis of modern physics.
It was the era of the establishment of modern physics by Isaac Newton.