Four names, four drinks, considerable overlap, and constant confusion. Here is what distinguishes them.
Cortado — Spain
Espresso cut with an equal volume of warm milk. Minimal foam. Served in a small glass, around 110 ml (3.7 oz).
The most coffee-forward of the four.
Piccolo latte — Australia
A ristretto with roughly twice its volume in milk, in a 90 to 100 ml (3 to 3.4 oz) glass, with a thin microfoam layer.
Milkier than a cortado, much smaller than a flat white.
Flat white — Australia and New Zealand
A double ristretto with around 130 ml (4.4 oz) of milk in a 150 to 175 ml (5 to 6 oz) cup, with a thin layer of glossy microfoam folded through.
The defining feature is the texture, not the size.
Gibraltar — United States
A cortado, named after the Libbey glass Blue Bottle served it in. Functionally identical to a cortado.
The rule of thumb
Ratio ascending: cortado (1:1), piccolo (1:2), flat white (1:3), latte (1:5 or more). A cappuccino sits near the flat white by ratio but carries a thick layer of foam on top instead of folded-in microfoam, which is the real difference between them.