Full Idea
I define a 'symbol' (of the predicate calculus) as either a bracket or a logical connective or a term or an individual variable or a predicate-letter or reverse-E (∃).
Gist of Idea
The 'symbols' are bracket, connective, term, variable, predicate letter, reverse-E
Source
E.J. Lemmon (Beginning Logic [1965], 4.1)
Book Reference
Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.139