A symbolic expression (also called s-expression) is a notation of a list-based tree. Inner nodes are lists of arbitrary length, outer nodes are primitive values (also called atoms) or the empty list.
A symbolic expression is either
An atom is a number, a string, or a symbol.
Symbolic expressions are used in programming languages like LISP or Scheme, where they denote both data structures and the program itself. This allows a LISP or Scheme program to process LISP or Scheme programs. That property is also used within Zettelstore.
Symbolic expressions are relatively easy to read, to parse, and to process.