Language Literals

What exactly is a “literal”? When we speak of a “literal”, do we speak of it as a language construct irrespective of its truth value? For example, is "~a" a literal as well as "a"? If not, would the term "proposition" be more apt when referring to the truth value of "literals"?

It is either an atom or the negation of an atom. An atom is the "smallest" formula in a logic-based language, namely something that can be either true or false. An atom can be equated to a proposition in propositional logic (logic without any variables and quantifiers). Most of the work in the [argumentation] literature is about propositional logic indeed, although some (including assumption-based argumentation) is not.