Hi
This writing is about an orthogonal matrix in linear algebra .
How did it get named orthogonal?
To know this, first of all, We need to know orthonormality.
Because this vector is composed of orthonormality component.
In vector, 
It is said to be in orthonormal state when it is in this state.
He doesn't react except for himself.
An internal value of 1 with oneself means that theta value is 90 degrees or 270.
i.e., That is why it was named this way.
These origins are useful for diagonalization.
Thank you for reading. happy new year