Tetragon or New Zealand Spinach
The name "New Zealand spinach" emphasizes both its closeness to this other vegetable and its southern origins.
While indeed found in New Zealand, tetragon also grows in China, Chile and a number of Atlantic islands. Its arrival in Europe dates back to the late 18th century, when it was introduced by Captain Cook's mission returning from New Zealand. Its cultivation then spread throughout the Anglo-Saxon countries.
The plant's fruit forms an odd 4-pointed capsule, which is responsible for its "geometric" name.
Tetragon is an annual plant of the Alzoaceae family, whose fleshy, thick green leaves are edible. Note that botanically it is unrelated to spinach, which belongs to the spinaca family.