One contemporary view on matter takes it as all scientifically observable entities whatsoever. Commonly, the definition is limited to such entities explored by physics.
The definition pursued here is of matter as whatever the smallest, most fundamental entities in physics seem to be. Thus matter can be seen as material consisting of particles which are fermions and therefore obey the Pauli exclusion principle, which states that no two fermions can be in the same quantum state. Because of this principle, the particles which comprise matter do not all end up in their lowest energy state, and hence it is possible to create stable structures out of fermions. In addition, the Pauli exclusion principle insures that two pieces of matter will not occupy the same location at the same time, and therefore two pieces of matter in which most energy states are filled will tend to collide with each other rather than passing through each other as with energy fields such as light.
In response to different thermodynamic conditions such as temperature and pressure, matter can exist in different "phasess", the most familar of which are solid, liquid, and gas. Others include plasma, superfluid, and Bose-Einstein condensate. When matter changes from one phase to another, it undergoes what is known as a phase transition, a phenonmenon studied in the field of thermodynamics.
"Matter" is also, apparently, used in contrast to form, in the sense of content.