substitution

Substitution is the act of replacing one element with another within a system, structure, or process. The term is used in many fields, including chemistry, mathematics, linguistics, genetics, and information security. It emphasizes the relation between what is replaced and the replacement.

Chemistry uses substitution reactions when an atom or functional group in a molecule is replaced by a different atom or group. These reactions can be nucleophilic or electrophilic. Examples include the substitution of a hydrogen atom in methane by chlorine and the nitration of benzene. In metal complexes, ligand substitution describes replacement of one ligand by another.

In mathematics, substitution refers to methods for replacing an unknown with an expression in terms of another variable. The substitution method solves systems of linear equations by solving one equation for a variable and substituting into the other. In calculus, a related idea is u-substitution, a change of variables to simplify integrals.

In cryptography, substitution ciphers replace each plaintext symbol with a corresponding ciphertext symbol according to a fixed rule. Classic examples include monoalphabetic ciphers, where each letter maps to a single substitute, and polyalphabetic systems such as the Vigenère cipher, which use multiple mappings to increase security.

In biology, a substitution mutation occurs when a single nucleotide base is replaced by a different base. This can change an amino acid (missense), introduce a stop codon (nonsense), or be silent. Substitutions are categorized as transitions (purine to purine or pyrimidine to pyrimidine) or transversions.

Linguistically, substitution is a general term for replacing a linguistic unit with another. This can involve pronoun substitution for repeated nouns, or the use of synonyms or paraphrases in discourse.