Three meanings of the term 'S-expression'
"Symbolic expressions" or "S-expressions" are the basic data type in Lisp. Particularly, Lisp programs are S-expressions as well. The notion has immense importance in Lisp community.
Surprisingly, designers of recent Lisp dialects avoid the term "symbolic expression" or "S-expression". It is used once in Picolisp documentation; only twice, almost accidentally, in CLtL2, and it isn't used in CL Hyperspec or recent Scheme standards. Clojure, according to its web site "extends the code-as-data system beyond parenthesized lists (s-expressions) to vectors and maps." Only Newlisp documentation appear to regularly uses the term.
However, the term S-expression is still extensively used in daily communication and literature. Unfortunately, there is no universal, unique meaning. Inconsistent use is sometimes noted; for instance, in P. Siebel's "Practical Common Lisp." More frequently, it is ignored.
For J. McCarthy, S-expression is finite sequence consisting of dots and parentheses and symbols. The symbols, truly atomic, cannot be analysed on characters. For instance, S-expression (left.right) is sequence of five elements: (, left, ., right and ).
More often1, S-expression is seen as sequence of characters. In that meaning, S-expression (left . right) is sequence consisting of 14 elements.
The most usually2, S-expression is data structure, perhaps tree consisting of cons cells and symbols. In that meaning S-expression (left . right) is cons cell, containing adresses of symbols left and right in memory.
1 For instance, in E. Shaphiro, "Common Lisp - an interactive approach"; F. Turbak and D. Gifford, "Design concept of programming languages."
2 For instance, in J. and G. Sussman and H. Abelson, "SICP"; D. Touretsky, "Common Lisp - A gentle introduction to symbolic computation"; R. Finkel, "Advanced programming languages design".
Three Meanings of The Term 'S-expression.'