A subdiscipline of condensed-matter physics that focuses on the properties of solids in a size range intermediate between bulk matter and individual atoms or molecules. The size scale of interest is determined by the appearance of novel physical phenomena absent in bulk solids and has no rigid definition; however, the systems studied are normally in the range of 100 nanometers (10⁻⁷ meter, the size of a typical virus) to 1000 nm (the size of a typical bacterium).

Other branches of science, such as chemistry and molecular biology, also deal with objects in this size range, but mesoscopic physics has dealt primarily with artificial structures of metal or semiconducting material which have been fabricated by the techniques employed for producing microelectronic circuits. Thus it has a close connection to the fields of nanofabrication and nanotechnology. The boundaries of this field are not sharp; nonetheless, its emergence as a distinct area of investigation was stimulated by the discovery of three categories of new phenomena in such systems: interference effects, quantum size effects, and charging effects.