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A literal is "any notation for representing a value within source code"
(Contrast this with identifiers, which refer to a value in memory.)
Examples:
"hey" (a string)
false (a boolean)
3.14 (a real number)
[1,2,3] (a list of numbers)
(x) => x*x (a function)
/^1?$|^(11+?)\1+$/ (a regexp)
Some things that are not literals:
std::cout (an identifier)
foo = 0; (a statement)
1+2 (an expression)