Satisfiability and validity in the context of "Tautology (logic)"

In mathematical logic, a formula is satisfiable if it is true under some assignment of values to its variables. For example, the formula ${\displaystyle x+3=y}$ is satisfiable because it is true when ${\displaystyle x=3}$ and ${\displaystyle y=6}$, while the formula ${\displaystyle x+1=x}$ is not satisfiable over the integers. The dual concept to satisfiability is validity; a formula is valid if every assignment of values to its variables makes the formula true. For example, ${\displaystyle x+3=3+x}$ is valid over the integers, but ${\displaystyle x+3=y}$ is not.

Formally, satisfiability is studied with respect to a fixed logic defining the syntax of allowed symbols, such as first-order logic, second-order logic or propositional logic. Rather than being syntactic, however, satisfiability is a semantic property because it relates to the meaning of the symbols, for example, the meaning of ${\displaystyle +}$ in a formula such as ${\displaystyle x+1=x}$. Formally, we define an interpretation (or model) to be an assignment of values to the variables and an assignment of meaning to all other non-logical symbols, and a formula is said to be satisfiable if there is some interpretation which makes it true. While this allows non-standard interpretations of symbols such as ${\displaystyle +}$, one can restrict their meaning by providing additional axioms. The satisfiability modulo theories problem considers satisfiability of a formula with respect to a formal theory, which is a (finite or infinite) set of axioms.