substitution

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act or an instance of substituting.
• noun The state of being substituted.
• noun One that is substituted; a replacement.

from The Century Dictionary.

• noun In biology: The assumption by one organ of a function which was at one time performed by another organ. Thus the swim-bladder in fishes shows “that an organ originally constructed for one purpose, namely, notation, may be converted into one for a wholly different purpose, namely respiration.”
• noun The acquisition by an organ of a secondary function which, at first performed incidentally, may gradually become the chief function if the primary function becomes useless or is performed by another organ. Thus “the little folds of skin which originally served as ovigerous frena, but which, like-wise, very slightly aided the act of respiration, have been gradually converted by natural selection into branchiæ, simply through an increase in their size and the obliteration of their adhesive glands.”
• noun In Scots law, a technical enumeration of a series of heirs.
• noun In civil law, the appointment, in a will, of a successor to a devisee or legatee; subrogation.
• noun The act of substituting, or putting (one person or thing) in the place of another; also, the state or fact of being substituted.
• noun The office of a substitute: delegated authority.
• noun In grammar, the use of one word for another; syllepsis.
• noun In Roman law, the effect of appointing a person to be heir, in case the heir first nominated would not or could not be heir.
• noun In French law, a disposition of property whereby the person receiving it, who is called the institute (le grévé), is charged either at his death or at some other time to deliver it over to another person called the substitute (l'appelé).
• noun In chem., the replacing of one or more elements or radicals in a compound by other elements or radicals.
• noun In algebra: The act of replacing a quantity by another equal to it; also, in the language of some algebraists, the replacement of a set of variables by another set connected with the first by a system of equations equal in number to the number of variables in each set. See transformation (which is the better term).
• noun The operation of changing the order of a finite number of objects, generally letters, that are in a row, the change following a rule according to which the object in each place is earned to some definite place in the row, this operation being regarded as itself a subject of algebraical operations.
• noun A linear transformation.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of substituting or putting one person or thing in the place of another
• noun The state of being substituted for another.
• noun rare The office or authority of one acting for another; delegated authority.
• noun (Civil Law) The designation of a person in a will to take a devise or legacy, either on failure of a former devisee or legatee by incapacity or unwillingness to accept, or after him.
• noun (Theol.) The doctrine that Christ suffered vicariously, being substituted for the sinner, and that his sufferings were expiatory.
• noun (Chem.) The act or process of substituting an atom or radical for another atom or radical; metathesis; also, the state of being so substituted. See Metathesis.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun the act of substituting or the state of being substituted
• noun a substitute or replacement
• noun chemistry (especially in organic chemistry) the replacement of an atom, or group of atoms, in a compound, with another

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the act of putting one thing or person in the place of another:
• noun an event in which one thing is substituted for another