The optical axis refers to an imaginary line around which the light waves travel. It is a vital concept in the field of optics used to describe the direction of light passing through a lens or an optical system. Other terms that can be used interchangeably with optical axis include focal axis and principal axis. The axis is an essential component in determining the position of images seen through an optical lens, with deviations from the axis resulting in distortion of the image. Additional related terms include image plane, nodal point, and optical center. In summary, the optical axis is a fundamental concept in optics, and understanding its synonyms is essential in mastering the topic.