Nanotechnology means technology with nanomaterials. Nanomaterials are defined as materials with at least one dimension of them of a nano size. A nano meter (nm) is an SI (Systéme International d’Unités) unit of length 10^-9 m or a distance of one-billionth of a meter. At this exceedingly small size, you are talking about a few/finite number of atoms or molecules. Tubes have two dimensions, a diameter, and a length, and out of these two dimensions, the length dimension will be much greater than the other one. And in nanomaterials the least dimension will be of nano size. Hence, nanotubes will have their diameters of nano size. Carbon nanotubes are made up of carbon atoms. They can be considered miniature cylinders. At present only carbon nanotubes are known in the class of nanotubes. Hence, the rest of the book will mention carbon nanotubes as simply nanotubes.