What is a unit matrix?

The unit matrix is every ##n##x ##n## square matrix made up of all except for the elements of the main diagonal that are all ones.
For example:
It is indicated as ##I_n## where ##n## representes the size of the unit matrix.
The unity matrix in linear algebra works a little bit like the number 1 in normal algebra so that if you multiply a matrix by the unit matrix you get the same initial matrix!