What term refers to the degree of sharpness of an antenna beam?

The term that refers to the degree of sharpness of an antenna beam is directivity. Directivity is a measure of how focused the antenna's radiation pattern is in a specific direction compared to an isotropic radiator, which radiates equally in all directions. A highly directive antenna has a narrow beamwidth, meaning it concentrates its energy in a certain direction, resulting in stronger signals in that area and reduced signals elsewhere. This characteristic is important in applications that require precise targeting, such as in communication systems where signals need to be sent or received over long distances.

The other terms mentioned do not specifically address beam sharpness. Gain refers to the overall increase in power that an antenna provides in a given direction compared to a reference. Resistance relates to the opposition to the flow of electric current and does not apply to the physical shape or focus of an antenna's radiation pattern. Waveform describes the shape of the signal being transmitted or received, which is distinct from how focused the signal is in space. Thus, directivity is the most appropriate term for the degree of sharpness of an antenna beam.