Common uses of the terms 'resolution' and 'depth of focus' (DOF) are explored as they relate to semiconductor lithography. A definition of DOF is given which is most appropriate to photolithography for IC manufacturing. Examples of the use of the definition for DOF for studying trends in lithography are given. Resolution is then defined based on realistic requirements for semiconductor manufacturing. Using this definition of resolution, the common scaling law of resolution with numerical aperture is shown to be inaccurate under typical conditions.