"Second-order approaches". What does this mean?

OK. Machine learning. In this context a 'second-order' approach to a problem appears to be a statistics/mathematics term. I am not qualified to say more than that. Maybe ask in Mathematics or Computer Science Stack Exchange?

In mathematics and logic

Second-order approximation, an approximation that includes quadratic terms

Second-order arithmetic, an axiomatization allowing quantification of sets of numbers

Second-order differential equation, a differential equation in which the highest derivative is the second

Second-order logic, an extension of predicate logic

In perturbation theory, a second-order perturbation may be obtained iteratively

https://en.wikipedia.org/wiki/Second-order

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