In ATC operations, what denotes the term "vertical separation"?

In air traffic control operations, "vertical separation" specifically refers to the altitude difference maintained between aircraft. This is a crucial concept in ensuring that aircraft are safely spaced in the vertical dimension during flight. Maintaining adequate vertical separation is vital for preventing collisions as it allows multiple aircraft to operate in the same airspace at different altitudes, which is particularly important in busy airspaces.

In contrast, the other options describe different forms of spacing or separation that are not related to altitude. For instance, the horizontal distance between planes pertains to lateral separation, while the space on the runway for landings addresses runway management rather than altitude considerations. The distance between air traffic control towers has no direct relevance to the separation of aircraft in flight. Thus, the selection highlighting the altitude difference accurately captures the essence of vertical separation within ATC operations.