The phrase "accretion model" refers to a concept in which something gradually grows or accumulates over time by adding or merging with other smaller things. Full definition
The seeming abundance of rocky super-Earths lends support to the core accretion model of planet formation, in which small rocky bodies collide and clump together to grow into these objects. (newscientist.com)
We argue that the correlation between stellar properties and giant planet occurrence is strong supporting evidence of the core accretion model of planet formation. (arxiv.org)
One puzzling detail is why Jupiter seems to have a relatively light core of no more than 10 Earth - masses, given that the core accretion model suggests a likely value of 20 to 30 Earth - masses. (scientificamerican.com)