Size comparison and examples of "nanosized systems" of study in nanothermodynamics (from the 2009 Teacher's Guide to Nanothermodynamics, Northeastern University). [4]
In thermodynamics, nanothermodynamics or "small-systems thermodynamics" is the thermodynamical study of phenomenon and processes at the nanometer scale (nanoscale), in which systems are in the in size range of
$10^{-9} \,$ meters (nanosystems), often consisting of a countable number of particles (nanoparticles), each with a discernable structure. Significant aspects unique to nanosystems, from the thermodynamic point of view, include their high surface-to-volume ratio, in which "surface effects" become increasingly important with decreasing size, and “fluctuations”, such as temperature fluctuations, among others. Nanothermodynamics, in short, is the application of the principles and laws of thermodynamics and statistical mechanics to the prediction of the properties and performances of nanoscale structures in small molecular systems.

History
The question of the issue of the application of thermodynamics on the nanoscale began to emerge after the nucleation reaction was discovered in the early 1930s. [2] The term ‘nanothermodynamics’ was being used as a synonym for small-systems thermodynamics as early as 2000. [1]

Most sources, e.g. Ali Mansoori, attribute the dominant introduction of the term nanothermodynamics to the introduction of its use in the title of the 2001 article “Nanothermodynamics” by American chemist and biophysicist Terrell Hill; themed to be a shortened version of the older phrase “small system thermodynamics” used by Hill in 1963 to designate the subject of the thermodynamics and statistical mechanics of systems of colloidal particles, polymers, or macromolecules. [3]

References
1. Giebultowicz, Tom. (2000). “Nanothermodynamics: Breathing Life into an Old Model” (abstract), Nature, 408: 299-301.
2. Feshbach, H. (1987). "Article", Physics Today, 40, 9.
3. (a) Hill, Terrell, L., (2001). “Nanothermodynamics”, Nanoletters, 1, 111, 273.
(b) Wang, C.X. and Yang, G.W. (2005). “Thermodynamics of Metastable Phase Nucleation at the Nanoscale” (section 2: Nanothermodynamics), Materials Science and Engineering R, 49, 157-202.
(c) Mansoori, G. Ali. (2005). Principles of Nanotechnology: Molecular-based Study of Condensed Matter in Small Systems, (Hill, pg. 85; chapter 3: “Thermodynamics and Statistical Mechanics of Small Systems”, pgs. 84-114). World Scientific.
4. Ginder, Ryan, Shastry, Tejas and Ko, Hsiao-Yu. (2009). “A Teacher’s Guide to Nano-thermodynamics”, Northwestern University.