An irreducible representation of a group is a group representation that has no nontrivial invariant subspaces. For example, the orthogonal group has an irreducible representation on .
Any representation of a finite or semisimple Lie group breaks up into a direct sum of irreducible representations. But in general, this is not the case, e.g., has a representation on by
(1)

i.e., . But the subspace is fixed, hence is not irreducible, but there is no complementary invariant subspace.
The irreducible representation has a number of remarkable properties, as formalized in the group orthogonality theorem. Let the group order of a group be , and the dimension of the th representation (the order of each constituent matrix) be (a positive integer). Let any operation be denoted , and let the th row and th column of the matrix corresponding to a matrix in the th irreducible representation be . The following properties can be derived from the group orthogonality theorem,
(2)

1. The dimensionality theorem:
(3)

where each must be a positive integer and is the group character (trace) of the representation.
2. The sum of the squares of the group characters in any irreducible representation equals ,
(4)

3. Orthogonality of different representations
(5)

4. In a given representation, reducible or irreducible, the group characters of all matrices belonging to operations in the same class are identical (but differ from those in other representations).
5. The number of irreducible representations of a group is equal to the number of conjugacy classes in the group. This number is the dimension of the matrix (although some may have zero elements).
6. A onedimensional representation with all 1s (totally symmetric) will always exist for any group.
7. A onedimensional representation for a group with elements expressed as matrices can be found by taking the group characters of the matrices.
8. The number of irreducible representations present in a reducible representation is given by
(6)

where is the group order of the group and the sum must be taken over all elements in each class. Written explicitly,
(7)

where is the group character of a single entry in the character table and is the number of elements in the corresponding conjugacy class.
Irreducible representations can be indicated using Mulliken symbols.