In logic, mathematics, and computer science, a formal system is a formal grammar used for modelling purposes. Formalization is the act of creating a formal system, in an attempt to capture the essential features of a real-world or conceptual system in formal language.

For example, in some colleges the procedures taken against failing students have been set out as a formal system. This gives greater clarity, of course accompanied with a loss of flexibility. Procedures for immigration and naturalisation have tended over time towards more formal models.

In mathematics, formal proofs are the product of formal systems, consisting of axioms and rules of deduction. Theorems are then recognised as the possible 'last lines' of formal proofs. The point of view that this picture encompasses mathematics has been called formalist. The term has been used pejoratively. On the other hand, David Hilbert founded metamathematics as a discipline designed for discussing formal systems; it is not assumed that the metalanguage in which proofs are studied is itself less informal than the usual habits of mathematicians suggest. To contrast with the metalanguage, the language described by a formal grammar is often called an object language (i.e., the object of discussion - this distinction may have been introduced by Carnap).

It has become common to speak of a formalism, more-or-less synonymously with a formal system within standard mathematics invented for a particular purpose. This may not be much more than a notation, such as Dirac's bra-ket notation.

Mathematical formal systems consist of the following:

1. A finite set of symbols which can be used for constructing formulae.
2. A grammar, i.e. a way of constructing well-formed formulae out of the symbols, such that is possible to find a decision procedure for deciding whether a formula is a well-formed formula (wff) or not.
3. A set of axioms or axiom schemata: each axiom has to be a wff.
4. A set of inference rules.
5. A set of theorems. This set includes all the axioms, plus all wffs which can be derived from previously-derived theorems by means of rules of inference. Unlike the grammar for wffs, there is no guarantee that there will be a decision procedure for deciding whether a given wff is a theorem or not.