Definitive Ephemeris is used for geometrically correcting Landsat data and provides improved accuracy over predicted ephemeris. An ephemeris is a set of data that provides the assigned places of a celestial body (including a manmade satellite) for regular intervals.
In the context of Landsat 7, ephemeris data shows the position and velocity of the spacecraft at the time imagery is collected, and has been refined to show the position and velocity of Landsat 7 in one-minute intervals.
Definitive Ephemeris files for both Landsat 7 and Landsat 5 are available to download.
Section 11.3.7 of the Landsat 7 Data Users Handbook also provides more details on definitive ephemeris.