A light month (also written light-month) is a unit of length. It is defined as the distance light travels in an absolute vacuum in one full month (thirty days of 86,400 seconds each) or 777,062,051,136,000 metres (~777 Tm). Note that this value is exact, since the metre is actually defined in terms of the speed of light.
The light month is not 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. There are, however, some exceptions. The Oort cloud, for example, is thought to extend between 10 and 20 light-months out from the Sun.
See also
