substitution
1. A particular kind of mapping on formal languages. Let Σ₁ and Σ₂ be alphabets. For each symbol a in Σ₁ let s(a) be a Σ₂-language. The function s is a substitution. A homomorphism occurs where each s(a) is a single word. s is Λ-free if no s(a) contains the empty word.

The function s can be extended to map Σ₁-words to Σ₂-languages: s(a₁…a_n) = s(a₁)…s(a_n)

i.e. the concatenation of the languages s(a₁),…,s(a_n). s can then be further extended to map Σ₁-languages to Σ₂-languages: s(L) = {s(w) | w ∈ L}

s(L) is called the substitution image of L under s.

2. See substitution cipher.

616. Substitution

1. Arsinoë put her own son in place of Orestes; her son was killed and Orestes was saved. [Gk. Myth.: Zimmerman, 32]
2. Barabbas robber freed in Christ’s stead. [N.T.: Matthew 27:15–18; Swed. Lit.: Barabbas ]
3. Canty, Tom young beggar takes to throne in prince’s stead. [Am. Lit.: The Prince and the Pauper ]
4. Edward, Prince of Wales kingling becomes urchin in clothing exchange. [Am. Lit.: The Prince and the Pauper ]
5. George, Tobey after Marcus’s death, replaces him in his family. [Am. Lit.: The Human Comedy ]
6. Hagar thinking herself barren, Sarah offers slave to Abraham. [O.T.: Genesis 16:1–4]
7. Leah deceptively substituted for Rachel in Jacob’s bed. [O.T.: Genesis 29:22–25]
8. whipping boy surrogate sufferer for delinquent prince. [Eur. Hist.: Brewer Note-Book, 942]

substitution (in genetics) A point mutation in which one base pair in the DNA sequence is replaced by another. There are two types of substitution mutation: transition mutations, in which a pyrimidine base (i.e. thymine or cytosine) is replaced by another pyrimidine base or a purine base (adenine or guanine) is replaced by another purine base; and transversion mutations, in which a pyrimidine base is replaced by a purine base, or vice versa. Most substitutions tend to alter one amino acid in a protein chain, which may or may not affect the functioning of that protein. Sickle-cell anaemia is an example of a substitution mutation in which thymine is replaced by adenine in the triplet coding for the sixth amino acid in the β-chain of haemoglobin.

substitution The replacement of one nucleotide in a DNA sequence by another as a result of a mutation event. See also nonsynonymous substitution, silent substitution, and synonymous substitution.

substitution (sub-sti-tew-shŏn) n. (in psychoanalysis) the replacement of one idea by another: a form of defence mechanism.