A signal in which the physical quantity used for transmission varies in an unquantized way according to the value of a physical quantity of the same or different nature.
Reference: I.E.C. (extract)
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
Celsius (previously Centigrade)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM