Holometabolism, also called complete metamorphism, is a form of insect development which includes four life stages - as an embryo or egg, a larva, a pupa and an imago or adult. In some species the holometabolous life cycle prevents larvae from competing with adults because they inhabit different ecological niches. Accordingly, their morphology can be adapted to just one phase of activity, such as larvae feeding for growth and development, as opposed to adults flying for dispersal and seeking new supplies of food for their offspring. Conversely, in some insects, the adults can protect and feed the younger stages.
Definition from Wikipedia: http://en.wikipedia.org/wiki/Holometabolism