substritution

Substitution is the act of replacing one element with another. The term appears in several disciplines, usually involving a replacement that preserves some structure or function of the original.

In mathematics and logic, substitution refers to replacing a variable or symbol with an expression or value. The substitution method solves systems of linear equations by solving one equation for one variable and substituting it into the others. In calculus, substitution, often called u-substitution, changes the variable of integration to simplify the integral.

In chemistry, a substitution reaction is one in which an atom or group in a molecule is replaced by a different atom or group. Substitution reactions are central to organic synthesis and include mechanisms such as SN1 and SN2, which describe different pathways and rates depending on the substrate and conditions.

In cryptography and computer science, substitution refers to replacing plaintext with ciphertext according to a fixed rule. Substitution ciphers, such as the Caesar cipher, map each symbol to another in a reversible way, forming the basis of classical cryptography. In modern computing, substitution can also mean string replacement operations or the use of substitution boxes (S-boxes) in encryption algorithms.

In linguistics, substitution describes replacing a linguistic element—such as a word, phoneme, or morpheme—with another to avoid repetition, satisfy phonotactic constraints, or encode information.