A long black is a double espresso poured over 100 to 120 ml (3.4 to 4 oz) 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 (4 to 7 oz), a long black 100 to 120 ml (3.4 to 4 oz). 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 traveled with the same wave of specialty coffee.
Making one
Fill a 180 ml (6 oz) 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 specialty — is a double ristretto in a 160 ml (5.4 oz) cup with textured milk, which is essentially a stronger, smaller flat white.
Ordering one in the US
Few US menus list a long black. Ask for a double espresso over a little hot water, or an americano with less water, and say water first. At home, water just off the boil, around 90 to 95 °C (194 to 203 °F), keeps the drink hot without scalding the crema.