What is defined as an empty space without matter?

A vacuum is defined as an empty space devoid of matter. In physics, the concept of a vacuum is often referred to when discussing scenarios where there are no particles present, meaning there is no air or any other matter in that space. This idea is commonly applied in areas such as space, where vast regions can be considered nearly perfect vacuums due to the extremely low density of particles.

In contrast, the other terms have specific meanings that do not align with the definition of a vacuum. A medium refers to a substance that can carry waves, such as air or water, and is inherently composed of matter. The atmosphere refers to the layer of gases surrounding a planet, which is certainly not empty. A field, in physics, usually refers to a concept like gravitational or electromagnetic fields, which describe forces acting in a space that can contain matter. Therefore, the correct answer highlights the unique characteristic of a vacuum as being the absence of any matter.