In our business this generally refers to an audio source that is directly in front of a listener or a transducer such as a microphone. This is at the 0 degree axis in a polar pattern. A microphone will generally produce the “truest” results if the desired source is on-axis (oriented directly in front of the sound source), although some creative engineers have been known to get desirable sounds by using a microphone’s off-axis response. For loudspeakers the meaning is similar – when the listener is directly on axis with a speaker he/she will be exactly in front of it. How a speaker’s characteristics change as the listener moves more off axis is an important part of the overall response.