identity matrix

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

• noun A square matrix with 1's along the diagonal from upper left to lower right and 0's in all other positions, constituting the identity element for matrix multiplication.

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

• noun linear algebra A diagonal matrix all of the diagonal elements of which are equal to 1.

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

• noun a scalar matrix in which all of the diagonal elements are unity