Planetesimal is a term used in the currently accepted model of Planet Formation: the combining of 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
Protoplanetary Disk (PPD)