The American Heritage dictionary: The science and technology of nanoscale devices and materials, such as electronic circuits, constructed using single atoms and molecules.
NASA: By definition, nanotechnology, or “nano”: is the creation of functional materials, devices, and systems through control and manipulation of matter on the nanometer length scale (1-100 nanometers). For the sake of comparison, an ordinary sheet of paper is 100,000 nanometers thick. At this scale, engineers have the ability to exploit novel phenomena and material properties, be they physical, chemical, biological, mechanical, or electrical.
Wikipedia: is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal of precisely manipulating atoms and molecules for fabrication of macroscale products, also now referred to as molecular nanotechnology. A more generalized description of nanotechnology was subsequently established by the National Nanotechnology Initiative, which defined nanotechnology as the manipulation of matter with at least one dimension sized from 1 to 100 nanometers. This definition reflects the fact that quantum mechanical effects are important at this quantum-realm scale, and so the definition shifted from a particular technological goal to a research category inclusive of all types of research and technologies that deal with the special properties of matter which occur below the given size threshold.