Atomic, molecular, and optical physics is the study of matter-matter and light-matter interactions on the scale of single atoms or structures containing a few atoms. The three areas are grouped together because of their interrelationships, the similarity of methods used, and the commonality of the energy scales that are relevant.
Atomic physics is distinct from nuclear physics, despite their association in the public consciousness. Atomic physics is unconcerned with the nuclear processes studied in nuclear physics, although properties of the nucleus can be important in atomic physics (e.g., hyperfine splitting).
Molecular physics focuses on multi-atomic structures and their internal and external interactions with matter and light.
Optical physics is distinct from optics in that it tends to focus, not on the control of classical light fields by macroscopic objects, but on the fundamental properties of optical fields and their interactions with matter in the microscopic realm.
All three areas include both classical and quantum treatments.