Mentre is an important conjunction in Italian that means ‘during the time that‘ or ‘at the same time as‘. It comes from the Latin dum interim via the old Italian domentre. In most cases, it translates as while (or whilst if you live in Britain) or as in English.
Mentre stavo facendo la spesa, ho incontrato un amico d’infanzia che non vedevo da anni.
While I was out shopping, I ran into a childhood friend I hadn’t seen in years.
Mentre stavo uscendo di casa, ha squillato il telefono.
As I was leaving the house, the phone rang.
You may also encounter the alternative form mentre che (lit: while that) or very rarely, mentreché written as one word.
Just like the English word while, mentre can function as a conjunction that highlights a contrast between two statements. When used in this sense, whereas becomes another possible translation.
A me piace uscire con gli amici, mentre a lei piace stare a casa per conto suo.
I like going out with friends, whereas she likes being at home on her own.
In order to add extra emphasis, Italians often place the word invece (instead, on the contrary) after mentre.
Lo credevo un po’ scemo mentre invece è molto intelligente.
I thought he was a bit of a fool whereas in actual fact he is very intelligent.
In old and literary Italian, mentre can also mean as long as / so long as.
An expression in which mentre appears as a noun rather than a conjunction is in questo / quel mentre which means at this / that moment.
Sono corso di sopra ed era in quel mentre che ho sentito un forte scoppio.
I ran upstairs and it was at that moment I heard a loud bang.
Nel mentre che, on the other hand, is a rarely used expression that also means while in both the temporal and contrasting sense. It appears far more frequently in its abbreviated form mentre che.
(Nel) mentre che tornavo a casa, è cominciato a piovere.
While I was going home, it started to rain.