The data objects of the language are called terms. A term is either a constant, a variable, or a compound term.
A constant is either a number (integer or floating-point) or an atom. Constants are definite elementary objects, and correspond to proper nouns in natural language.
Variables and compound terms are described in ref-syn-trm-var, and ref-syn-cpt, respectively.
Foreign data types are discussed in the context of library(structs);
see lib-structs.