A galactic tide is the result of a tidal force imposed by a galaxy. If two galaxies pass sufficiently close (a galaxy collision or just a close encounter) their gravity can affect orbits of stars in the other (galaxy interaction). For example, disruptions such as a tidal arm or tidal tail can result.
The phrase galactic tide has also been used to describe effects within the galaxy. For example, the Oort Cloud is presumed to be disturbed a bit when the Sun's orbit passes closest to the galactic center, possibly resulting in long-period comets passing through the inner solar system.