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Given a square $n$-th order matrix, say $A=[a_{ij}]$,

  • $a_{ii}$'s are referred to as diagonal entries
  • $a_{ij}$'s, $i\neq j$ are referred to as off-diagonal entries

What about in particular $a_{ij}$ with $i

  • strictly upper triangular entries
  • upper diagonal entries

But it seems I couldn't find it used anywhere on the internet. I could be wrong...

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    Lower and upper triangulation of a matrix: https://en.m.wikipedia.org/wiki/LU_decomposition – 2017-02-15
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    @TroyWoo: Do you mean Sub and Super-Diagonal http://mathworld.wolfram.com/Superdiagonal.html? – 2017-02-15
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    @Moo Not only that, also the entries above superdiagonal entries. What do you call that? – 2017-02-15
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    @Moo In light of the LaTeX grammar, we may call them supersuperdiagonal...Just kidding... – 2017-02-15

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People sometimes refer to "the entries above the main diagonal" of a matrix. For example, https://mathematica.stackexchange.com/questions/41362/how-to-select-all-elements-above-the-main-diagonal-of-matrix

One also sometimes sees a mention of the elements below the main diagonal.

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    Thanks...LOL, I can't ... can I just call them nilpotent entries... – 2017-02-15