Figura etymologica is a rhetorical figure in which words with the same etymological derivation are used in the same passage. To count as a figura etymologica, it is necessary that the two words be genuinely different words and not just different inflections of the same word. For example, the sentence Once I loved, but I love no more is not a figura etymologica since although love and loved are obviously etymologically related, they are really just inflections of the same word.
Examples in modern English are the phrases "might and magic" (both of which are derived from the Indo-European mag(h)) and "chai tea," in which both come from words for tea (cha and te) in different Chinese dialects.
In fact, the figura etymologica has been both much more broadly and narrowly defined. In the narrowest definition, it is restricted to specialized uses of the accusative with cognate verbs (for example, live a good life, sing a long song, die a quiet death). In modern linguistics, this same construction goes by the name of "cognate object construction" (COC). In its less restricted sense, the figura etymologica refers to just about any sort of repetition of cognate words relatively close to each other.