Stufe (=level) of a non-real field (wikipedia.dewikipedia.de). It is the least number of squares $a_i^2$ such that $\sum_i a_i^2 = -1$, $\infty$ if no such sum exists.
In this paperthis paper, the level of a subgroup of $SL_2(\mathcal{O})$ is defined ($\mathcal{O}$ a number field), as the generalisation of the stufe of a field, so the term has been translated, but only in a shift of context.
To pick a random paper, try The stufe of number fieldsThe stufe of number fields.