What term describes a large body of air that has uniform temperature and moisture content?

The term that describes a large body of air with uniform temperature and moisture content is "air mass." An air mass forms when air remains over a particular region for a long enough period, allowing it to acquire the characteristics of the underlying surface. These characteristics can include temperature, moisture, and ultimately, the weather associated with that specific air mass.

Air masses can be classified based on their source regions—such as continental or maritime, and polar or tropical—which determine their temperature and moisture content. For example, a maritime tropical air mass would be warm and moist, whereas a continental polar air mass would be cold and dry.

The other terms mentioned, while related to meteorology, do not accurately capture the definition of a large body of air with uniform characteristics. Air pressure refers to the force exerted by the weight of air above a given point, and atmospheric fronts are boundaries between different air masses that can lead to weather changes. Lastly, a weather system encompasses the interactions between various atmospheric components but is broader than just an individual air mass. Therefore, "air mass" distinctly fits the definition provided in the question.