On-axis response

On axis when referring to microphones is normally just referred to as the microphone's frequency response.

When referring to microphone placement in a recording environment on-axis is a reference to direct positioning of a microphone at a sound source. In other words if a microphone is pointed off-axis at a sound source it means the microphone is not pointed directly at the source.

With speakers the on-axis response is measured directly in front of its cabinet. The on-axis response of a speaker is generally the best (or flattest) response that a speakers produces.