Planetesimal is a term used in the currently accepted model of Planet Formation: the combining of Cosmic Dust into small objects, with additional combination leading to planets. When the object is large enough to be held together by its own Gravity, it is termed a planetesimal. In the solar system this would be an object roughly at least 1 km diameter. It is also assumed that its motion is dominated by a gravitational orbit rather than by the influence of gas it is passing through.
Kilometer Size Barrier