Estuaries, rias, fjords, coastal lagoons, bahiras, river mouths, tidal creeks, deltas and similar coastal environments are often regarded as a single broad conceptual class. ‘Brackish’, ‘estuarine’, ‘paralic’ and ‘transitional’ are terms used in different contexts to designate collectively this class of environments. Nevertheless every term, generated from different historical perspectives and scientific points of view, excludes some of the above-mentioned environments. These terms and definitions were examined with regard to their meaning and history. The main attributes have been extracted from definitions and arranged in a conceptual scheme giving an overall direct perception of their relationships. This analysis provided evidence for the occurrence of two major groups of attributes: hydrological and geomorphic. Although the significance of hydrological attributes is instantly recognisable, geomorphic attributes imply subjacent concepts of geographical scale and hydrological features not expressly formulated in definitions, such as a limited supply of seawater to the system.