Literal (Boolean)

A literal is an appearance of a variable in a Boolean expression, either complemented or uncomplemented. Each occurrence counts separately, even if it’s the same variable.

In $B(A+D)+AC$, there are five literals: $A$ (twice), $B$, $C$, $D$. The expression $\overline{A}B$ has two literals: $\overline{A}$ and $B$.

The literal count is the simplest measure of expression complexity. When minimizing a Boolean function (using a Karnaugh Map or Boolean Algebra identities), one goal is reducing literals, because each literal corresponds to a wire/transistor connection in the resulting circuit.

A complemented literal like $\overline{A}$ requires an inverter on the input $A$. So the count of distinct complemented literals also matters for Gate Input Cost: every complemented variable adds one NOT gate.

Don’t confuse “literal” with “variable” or “term.” A variable like $A$ exists once in your design; literals are how many times it appears in an expression. A term is a literal-or-conjunction-or-disjunction of literals, a structural unit of the expression. Boolean Function covers how literals and terms combine into standard forms.