A substitution cipher

(chemistry) a reaction in which one functional group is replaced with another

(chemistry) in aliphatic organic chemistry, a form of substitution reaction in which the leaving functional group is replaced with a nucleophile

(chemistry) in aromatic organic chemistry, a form of substitution reaction in which the leaving functional group (normally hydrogen) is replaced with an electrophile

(cryptography) A method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.

A document in which the party to a lawsuit states that his/her attorney of record is being substituted for by another attorney or by the party acting for himself/herself (in propria persona).

(linear algebra) A method of solving linear systems that have been transformed into row echelon form. / (combinatorics) A method of solving recurrence relations.

plural of back substitution

(sports) A rule in some sports that allows players to enter and leave the game for other players many times during the course of the game, and for coaches to bring in and take out players an unlimited number of times.

plural of free substitution