Baleno (masculine) is one way of saying flash or flare in Italian. It derives from the verb balenare which means to flash (in a literal sense) or to pop into one’s head (in a figurative sense).
By extension, it can be used to refer specifically to a flash of lightning, though the synonyms lampo, fulmine and saetta are more common.
Important: Make sure not to confuse baleno with the similar sounding balena which means whale!
Where baleno appears most frequently is in the prepositional phrase in un baleno or alternatively in un battibaleno, both of which mean in a flash or in the blink of an eye.
Gli uccelli sono volati via in un battibaleno.
The birds flew away in a flash.
Figuratively speaking, it can also refer to the rapid appearance of something such as il baleno di un sorriso the flash of a smile) or il baleno di uno sguardo (the quick glance).
It also appears in the proverb << Dopo il baleno viene il tuono, dopo le minacce vengono i fatti >> which translates as << After the lightning comes the thunder, after the threats come the facts >>.