When two air masses with different properties move out of their source regions and come into contact, what is the zone between them called?

Why: The Remote Pilot Study Guide states, "As air masses move out of their source regions, they come in contact with other air masses of different properties. The zone between two different air masses is a frontal zone or front. Across this zone, temperature, humidity and wind often change rapidly over short distances."

• Confuses a front with a pressure gradient, which describes the rate of change of atmospheric pressure over distance and drives wind, not the boundary zone between two air masses.
• Confuses a front with a standing lenticular cloud, a lens-shaped cloud formed on the crest of a mountain wave that indicates turbulence — an unrelated phenomenon the guide describes separately.