Google translate gives me "en virto de".
If this is correct, could you please give me an example sentence in Esperanto?
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In English by virtue of signals a reason, that reason coming after of in the form of a noun phrase (similar to because of, due to, etc.).
A similar construction in Esperanto, introducing a noun phrase as a reason, would be pro.
Pro la multon da tempo, kiun ŝi elspezis helpante al Roberto fariĝi virino, ŝi forgesis forigi la ungolakon.
'With all the time she spent helping Robert become a woman, she forgot to take off her nail polish.'
As Eduardo Trápani already implied single word requests without a context are often quite hard to answer, especially when it comes to idioms like this.
Virtue as such means behaviour showing high moral standards or an order of celestial hierarchy in Christian mythodology. The saying "by virtue of" was originally based on the former meaning. That there exist "a higher standard" that gives a king, a nobleman or some self-promoted person a right or a justification to do something. Later the expression has shifted to mean "because of" or "due to":
The Esperanto word virto refers to that high moral standard. For more specific definitions and examples of use see PIV. The Esperanto word virto has nothing to do with a right or a justification. The preposition pro is one possibility, but there are others depending on the context. Remember, translate ideas, not words.
So if I were to translate the examples above
At: https://eo.wiktionary.org/wiki/Aldono:Vortaro_angla-Esperanto_v the examples under 'virtue' might be helpful.