A free particle is a physical object that is not confined by any external force or field. It moves independently and is not subject to any external constraints. Some synonyms for free particle include unbound particle, independent particle, and unconstrained particle. These terms describe the same concept of a particle that is not influenced by any outside forces or fields. Other related terms include free movement, free motion, and free path. Free particles are important in various fields of physics, including chemistry, optics, and quantum mechanics. Understanding the behavior and properties of free particles can provide insight into the behavior of larger physical systems and phenomena.