Planetesimal is a term used in models of planet formation, including the currently most-accepted ones: the combining of dust into small (solid) objects, with additional combination leading to planets. When the object is sufficiently large 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.
Some solar system objects of that size, including some comets and asteroids, are presumed to be left-over planetesimals.