Dilution is what I like to call the general effect in translation of flattening, watering down, blunting the impact, or erasing the tropes of a poetic text.
It is a similar concept to Antoine Berman's term "Qualitative Impoverishment" ("Translation and the Trials of the Foreign," in Venuti's Translation Studies Reader), but my term encompasses a wider range of effects. I view dilution as a general tendency in translation in general, even in otherwise good translations. In other words, translation in general tends to go in this direction, and readers generally tolerate this. I will use some examples from Christopher Johnson's recent Quevedo translation (Selected Poetry of Francisco de Quevedo ), published by the U of Chicago P. This is not a bad translation at all, in the sense that it is both readable and accurate. It comes recommended by very highly regarded specialists. I recommend that you have your local library buy this book. Another disclaimer is that Quevedo is the preeminent poet of verbal wit. His effects are not inherently "translatable" in the conventional sense. There are other values in translation than preserving the original poetic force of the text. Readability, "fluency," rhythmic grace, etc... A translation that didn't dilute might be much less acceptable to many readers.
My point is that we should know what it is we're missing. Hence my nitpicking comments, beginning with several things in the first poem in the book, the famous sonnet beginning "Ah de la vida..."
'Aquí de los antaños que he vivido." "Help, here are the years I have lived."
There is a play on words: antaño means somethings like "yesteryear" or "formerly," whereas the translator treats it as though it were simply "años." Quevedo makes an adverb (antaño) into a plural noun.
"Falta la vida, asiste lo vivido" "Missing is life, existence remains"
"Lo vivido" means "what has been lived," the sum total of past life. Quevedo uses words from the same family (vida, vivido) to hammer home the antithesis. The word existence has no sense of pastness about it. The syntactic parallelism disappears, because the translator has inverted the word order in one phrase but not the other. The original is more vivid, with the verb asistir being the perfect complement to faltar. "Life skips class, the lived attends."
"y no hay calamidad que no me ronde" "and everywhere calamity awaits"
What Quevedo says is that "there is no calamity that doesn't lie in wait for me." That's a much stronger statement. Waiting (or awaiting) is much blander that the verb rondar. Here is means something closer to threaten. Good iambic pentameter, though.
"y he quedado / presentes sucesiones de difunto" "and so I succeed my dead self again." I think part of the verbal wit is in the plural noun "sucesiones." The self lives in a series of present moments, in each of which he is essentially a dead man. In the translation, this becomes a single event. In the sonnet beginning "Miré los muros de la patria mía" Quevedo creates a powerful structure by using a series of verbs in the preterit at the beginning of the first three stanzas: "Miré los muros," / "Salíme al campo" / "Entré en mi casa." The translator varies this, inserting some present participles, and dismantles the effect. In another poem translator uses the word "lesson" to translate "escarmiento," a much harsher word. It does mean "lesson," but in the sense that getting beaten up is a "lesson," in other words, a harsh punishment or very stern warning. I would suggest a word like "scourge."
Any one instance of dilution would be one thing; an overall pattern of dilution is quite another.