When we multiply out $(x + y)(x + y)$, we refer to the two $xy$ terms as "cross terms". Is there a corresponding term for the $x^2$ and $y^2$ terms?
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Depending on the context, "diagonal terms" might work:
the cross-terms are the off-diagonal terms in this quadratic form and the other ones are the diagonal terms.
Direct or straight might be what you are looking for, as opposed to cross, crossed or mixed (since each resultant term has either one variable to a power or two different variables, a "mixture").
I was also taught that you can multiply $(a+b)(c+d)$ using the acronym FOIL for First, Inside, Outside, Last (which is mixing sequential and spatial metaphors).
The univariate terms is unambiguous. I like 'pure' but am not sure how correct this is.