The term "heavier-than-air" refers to objects that have a greater weight than air and, therefore, require a force to keep them aloft. Some synonyms for this term would be "aerodynamically designed" or "flight-worthy." Another synonymous phrase might be "powered flying vehicles," which includes airplanes, helicopters, and rockets. In contrast, the term "lighter-than-air" refers to objects that are lighter than air and are lifted by helium or hot air, such as blimps and hot air balloons. Understanding these concepts is important in the fields of aviation and engineering, as they relate to the production of sophisticated machines and the science of aerodynamics.