Nanotube is a hollow nanofibre. Carbon nanotube is a nanotube composed of carbon nanotubes usually consist of curved graphene layers, including single-wall carbon nanotubes and multi-wall carbon nanotubes.
Reference : ISO/TS 27687:2008
A discrete hollow entity which has two dimensions of the order of 100 nm or less and one long dimension
Reference : Scientific Committee on Emerging and Newly Identified Health Risks 2007