From the Wikipedia

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The Short Definition

Physics is the Study of Matter and Energy.

Physics (from the Greek, phusikos, "natural", and phusis, "nature") is the science of Nature in the broadest sense. Physics deals with matter and energy and the fundamental forces of nature that govern the interactions between particles. Physicists study a wide range of physical phenomena, from the sub-nuclear particles of which all ordinary matter is made (particle physics) to the material Universe as a whole (cosmology).

Some of the properties studied in physics are common to all material systems, such as the conservation of energy. Such properties are often referred to as laws of physics. Physics is sometimes said to be the "fundamental science", because each of the other natural sciences (biology, chemistry, geology, etc.) deals with particular types of material systems that obey the laws of physics. For example, chemistry is the science of molecules and the chemicals that they form in the bulk. The properties of a chemical are determined by the properties of the underlying molecules, which are accurately described by areas of physics such as quantum mechanics, thermodynamics, and electromagnetism.