What defines a positive-displacement air compressor?

A positive-displacement air compressor is defined by its operation of moving a fixed volume of air with each cycle. This mechanism typically involves trapping a certain amount of air and then reducing the volume of that trapped air chamber, effectively compressing it. This type of compressor operates in a way that is independent of any variances in airflow demand, which is why it is characterized by this fixed volume movement.

In contrast, other types of compressors might focus on increasing the velocity of air (the first choice) or adjusting airflow rates dynamically (the third choice); however, those functions do not align with the principle of positive displacement. Additionally, while some compressors are utilized in jet engines, being specifically designed for that purpose (the fourth choice) does not define what a positive-displacement compressor is, as these compressors can be used in a variety of applications beyond aviation. Thus, the clarity in how a positive-displacement compressor functions aligns perfectly with the definition given in the correct answer.