Is there a specific terminology for these upper unitriangular matrices?

Question

I came across upper unitriangular $n \times n$ matrices of the following form (here for $n = 5$): $$ \pmatrix{ 1 & a & a^2 & a^3 & a^4 \\ 0 & 1 & a & a^2 & a^3 \\ 0 & 0 & 1 & a & a^2 \\ 0 & 0 & 0 & 1 & a \\ 0 & 0 & 0 & 0 & 1 } $$ Given their very simple shape, I wondered if they had a specific name. Even without a name, references would be welcome.