A physical quantity is a property of a material or system that can be quantified by measurement. It can be expressed as a value, which is the algebraic multiplication of a numerical value and a unit of measurement. Physical quantities can be defined either by specifying how they are measured or by stating how they are calculated from other measurements. For example, distance and time are defined by specifying methods for measuring them, whereas average speed is defined as distance traveled divided by time of travel.

The measurements of physical quantities are expressed in terms of units, which are standardized values. Without standardized units, it would be extremely difficult for scientists to express and compare measured values in a meaningful way. The units in which physical quantities are measured are organized in a dimensional system built upon base quantities, each of which is regarded as having its own dimension. The seven elemental physical quantities are temperature, mass, length, electrical charge, mole, luminosity, and time.

Physical quantities can be extensive or intensive. They are extensive if they depend on the mass or size of the object and intensive if they do not. For example, mass and volume are extensive physical quantities, while density and temperature are intensive physical quantities.

In summary, physical quantities are properties of materials or systems that can be quantified by measurement and expressed as a value with a unit of measurement. They are organized in a dimensional system built upon base quantities, and their measurements are expressed in terms of standardized units.