A light week (also written light-week) is an astronomical unit of length. It is defined as the distance light travels in an absolute vacuum in one week (seven days of 86,400 seconds each) or 181,314,478,598,400 meters (~181 Tm).
Note that this value is exact, since the metre is actually defined in terms of the speed of light. The light week isn't very frequently used at all since there are few astronomical objects of that magnitude; the orbits of outer solar system objects are better measured in light days or light hours, whilst interstellar distances are on the order of light years. The Oort cloud, for example, is thought to extend between 41 and 82 light-weeks out from the Sun.