Isolation mass is the mass of a planet-to-be after accretion of nearby planetesimals (objects with a diameter on the order of 1 km). Such accretion slows and virtually ceases when the number of such nearby planetesimals has sufficiently decreased. Current models generally estimate the isolation mass to be within the range of a Moon mass to a Mars mass, after which the planet-to-be fits the classification of oligarch.
Formation of isolation mass objects is a step in the nebular hypothesis of planet formation, where planetesimals (~1 km diameter) form by some means and the largest (oligarchs) accrete the others, the oligarchs' orbits eventually interfering and growing unstable, the oligarchs colliding and merging into rocky planets such as Mars and Earth. This model was developed before the detection and study of extra-solar planets, and since-discovered planetary systems add challenges to the concept. An isolation mass planet is a planet that adheres to the concept, and an isolation mass core is a planetesimal that has grown to the isolation mass, the name suggesting the possibly that it might grow further through impact or gas accretion.
The isolation mass is determined by the size to which the Hill radius can grow as the object grows, along with the density of material within that part of the protoplanetary disk, i.e., available for accretion. The Hill radius is based upon gravitation and there are other factors which contribute to the limits on accretion such as gas and radiation pressure; a stability radius that also taking these into account can be used for a more precise model.
Accretion depends upon the mass of the objects accreted, so independent isolation masses can be derived for separate mass ranges. The term pebble isolation mass refers to an isolation mass derived for pebble accretions, in this case referring to pebbles, objects in the range of roughly centimeter to roughly meter diameter.