What is a caper, exactly?

According to Wikipedia,

The caper (Capparis spinosa L.) is a perennial spiny shrub that bears rounded, fleshy leaves and big white to pinkish-white flowers. A caper is also the pickled bud of this plant. The bush is native to the Mediterranean region, growing wild on walls or in rocky coastal areas throughout. The plant is best known for the edible bud and fruit (caper berry) which are usually consumed pickled. Other species of Capparis are also picked along with C. spinosa for their buds or fruits.

The buds, when ready to pick, are a dark olive green and about the size of a kernel of maize. They are picked, then pickled in salt, or a salt and vinegar solution, or drained. Intense flavor is developed, as mustard oil (glucocapparin) is released from each caper bud. This enzymatic reaction also leads to the formation of rutin often seen as crystallized white spots on the surfaces of individual caper buds.