Particle
Also called Point particle
A particle is a body treated as having mass and a position but no size, shape or internal structure. AP Physics usually calls this an object; particle appears mainly where a system is built from several separate masses.
Older textbooks open mechanics with particle motion. The current AP framework mostly does not: its word for a body with no size is object, defined in College Board's own vocabulary appendix as a physical thing where the internal structure and properties of the thing are ignored.
Where the CED does reach for particle is counting. The Topic 2.1 boundary statement says AP Physics 1 only expects students to calculate the center of mass for systems of five or fewer particles arranged in a two-dimensional configuration, or for systems that are highly symmetrical. Unit 8 uses the word in its other sense, for the constituent particles of a fluid.
So read particle as a signal about the model, not a new quantity. It says: this thing has a mass and a location, and nothing else about it enters the calculation.