Great Journey of Life

The Great Journey of Life was what the Nimons told the Skonnans the name of their life cycle was. The Doctor and Romana revealed that it had nothing to do with perpetuating the actual biological life of the Nimons. Rather the cycle of invading a planet, stripping that planet of its resources, and departing for a new one merely maintained their lifestyle of incredibly high power usage. (DW: The Horns of Nimon)

The term may, in fact, have been deliberately mystical, so as engender the support of more primitive local species, who were necessary to help the Nimons achieve their goals. The Great Journey of Life is similar, in effect, to the life cycle of the Swarm, except that the Swarm's consumption of planets was actually biologically necessary.