Wikipedia says that the meaning of the phrase "‘arob", or "עָרוֹב", is disputed.
I have always been taught it as "flies" - and that's the word used by the NIVUK translation of Exodus 8:21. In the so-called Orthodox Jewish Bible, Shemot 8:21 it simply uses "arov", and in the so-called Complete Jewish Bible, Sh'mot 8:17 it says "swarms of insects".
Has there been any more recent interpretation of the word?
