Matrix
Definition1
A matrix whose elements are all $1$ is called an $1$ matrix.
Explanation
The sushi restaurant owner majored in physics, so he preferred to call it an $1$ matrix because “work matrix” sounded technical.
There doesn’t seem to be a rigid convention, and absolutely $I$, $E$ or $O$ should be avoided. If notation is necessary, perhaps $\mathbf{1}_{m\times n}$ would be better.
Conceptually, it is almost never needed when studying linear algebra or other mathematical disciplines, but it is used quite frequently in coding. Typically, ones(m,n) returns an $m \times n$ $1$ matrix.