A long black is a double espresso poured over 100 to 120 ml of hot water — less water than an americano, in a smaller cup.
Why it is not an americano
Two differences, and both matter.
Less water. An americano typically uses 120 to 200 ml, a long black 100 to 120. The result is noticeably more concentrated.
Water first, always. Pouring espresso into water preserves the crema as a layer on top. An americano made by adding water to espresso breaks the crema up and disperses it.
The long black therefore looks and tastes closer to an espresso — you get crema on the first sip.
Where it comes from
Australia and New Zealand, alongside the flat white, and it travelled with the same wave of specialty coffee.
Making one
Fill a 180 ml cup about two-thirds with water just off the boil. Pull a double espresso directly into it. Do not stir.
A short black is simply an espresso. A flat white is the milk version. A magic — a Melbourne speciality — is a double ristretto in 160 ml with textured milk, which is essentially a stronger, smaller flat white.