In Lisp 1.5 it is defined like this:
"An S-expression is either an atomic symbol or it is composed of these elements in the following order, a left parenthesis, an S-expression, a dot, an S-expression, and a right parenthesis."
But this is history. Today I'd describe them like this: s-expression is a list of atoms or s-expressions, where the first element of the list defines the meaning of the whole s-expression.
Note that symbol is an atom, but so are numbers, strings, characters and other objects (actually, anything that is not a list).
Note also that this description is heavily influenced by Common Lisp.
Some examples of Symbolic Expressions:
abc "foo" 42 (print 17) (list a b c (+ 1 2))
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