During the 50’s of the last century, at the start of the exploitation of nuclear fission for the production of electricity, the design and the construction and even the operation of the pioneering reactors occurred without any definition or technology framework for Nuclear Engineering (NE) and, inside NE, Nuclear Thermal-hydraulics (NTH). The needs from industry and the engagement of researcher during the following decades provided a shape and boundaries for NE and NTH. An attempt is made in this paper to put the basis for the definition of current NTH. Reference is made to a recent issue (published 2019) of the Journal Nuclear Engineering and Design. The attributes, the topics and the coupling of NTH with other disciplines belonging to NE are discussed in the paper. The generic fields of thermal-hydraulics and NTH are described; however, focus is given to NTH in Water Cooled Reactors (this is a synonymous of ‘thermal-hydraulics in Water Cooled Nuclear Reactors’) when selected topics and couplings are considered.