I hope someone can point me to a quick definition of the following terminology.


I keep coming across *wild* and *tame* in the context of classification problems, often adorned with quotes, leading me to believe that the terms are perhaps not being used in a formal sense. Yet I am sure that there is some formal definition.


For example, the classification problem for nilpotent Lie algebras is said to be wild in dimension $\geq 7$. All that happens is that in dimension $7$ and above there are moduli. In what sense is this wild?


Thanks in advance!



When is a classification problem "wild"?
I am not an expert but in the algebra and representation theory the apparently standard definition is [as follows](https://encyclopediaofmath.org/wiki/Representation_of_an_associative_algebra) (see also [Drozd - Tame and wild matrix problems](https://doi.org/10.1007/BFb0088467 "Drozd, J.A. (1980). Tame and wild matrix problems. In: Dlab, V., Gabriel, P. (eds) Representation Theory II. Lecture Notes in Mathematics, vol 832. Springer, Berlin, Heidelberg") and [Belitskii and Sergeichuk - Complexity of matrix problems](https://arxiv.org/abs/0709.2488v1 "Genrich R. Belitskii, Vladimir V. Sergeichuk. Complexity of matrix problems. 2007. doi:10.1016/S0024-3795(02)00391-9. zbMATH review at https://zbmath.org/?q=an:1030.15011")): a problem is wild if it contains a subproblem which is equivalent to the problem of simultaneously reducing to canonical form two linear operators on a finite-dimensional space.


