Fidelity (or "faithfulness") and transparency, dual ideals in translation, are often (though not always) at odds. A 17th-century French critic coined the phrase "les belles infidèles" to suggest that translations, like women, can be either faithful or beautiful, but not both.
Fidelity is the extent to which a translation accurately renders the meaning of the source text, without distortion.
Transparency is the extent to which a translation appears to a native speaker of the target language to have originally been written in that language, and conforms to its grammar, syntax and idiom. John Dryden (1631–1700) writes in his preface to the translation anthology Sylvae:
Where I have taken away some of [the original authors'] Expressions, and cut them shorter, it may possibly be on this consideration, that what was beautiful in the Greek or Latin, would not appear so shining in the English; and where I have enlarg’d them, I desire the false Criticks would not always think that those thoughts are wholly mine, but that either they are secretly in the Poet, or may be fairly deduc’d from him; or at least, if both those considerations should fail, that my own is of a piece with his, and that if he were living, and an Englishman, they are such as he wou’d probably have written.