(linear algebra) A diagonal matrix all of the diagonal elements of which are equal to 1.

(linear algebra) An identity matrix

(linear algebra) A matrix in which the entries outside the main diagonal are all zero.

(chemistry, physics) a technique for maintaining reactive molecules in an inert material at low temperature for spectroscopic study

(graph theory) A matrix showing the relationship between two classes of objects.

(linear algebra) Any n×n square matrix for which there exists a corresponding inverse matrix (i.e., a second (or possibly the same) matrix such that when the two are multiplied by each other, in either order, the result is the n×n identity matrix).

(graph theory) A matrix used to determine whether all of the edges of a graph can be traversed without visiting a vertex more than once.

(geology) The fine-grained background material in which crystals, pebbles or fossils are embedded in a sedimentary rock

(anatomy, cytology) All the connective tissues and fibres that are not part of a cell, but rather provide support.

(computing, also attributively) A two-dimensional array or pattern of dots used (for example, by a display device or a printer) to represent alphanumeric characters and pictures.