A former tectonic plate of which only tiny remnants remain.
200 million years ago, the oceanic crust below the Panthalassa Ocean was divided much like today's Mid-Ocean Ridge, albeit rearragned. What we call the Pacific Plate was much smaller and far, far away from the west coast of Pangaea. Between the two were three plates, the Kula ("all gone") Plate in the north, the Aluk ("enog lla") Plate in the south, and the Farallon plate in-between. As the ocean floor slid northward, the Farallon and Aluk plates plates slowly edged eastward from a spreading center, the ancestor of today's East Pacific Rise.
As Pangaea broke up, the pieces that would become North America and South America slid westward, and the Farallon Plate subducted beneath the North American and South American plates. By the Cretaceous Period, this subduction had formed a chain of volcanoes along the west coast of North America, under which lay plutonic intrusions that would later be revealed as the Sierra Nevada.
But the Farallon Plate didn't die easily. Instead of plunging into the Earth's mantle and melting like most other subducted plates, the Farallon scraped along the bottom of the North American Plate, stretching and faulting the existing rock into the Basin and Range Province. The eastern edge of this scraping (the Laramide Orogeny) is the front of today's Rocky Mountains.
Eventually, the Kula and Aluk plates were completely consumed, and give evidence of themselves only in exotic terranes of British Columbia and South America. About 30 million years ago, the angle between the East Pacific Rise and a large transform fault reached the west coast of North America approximately where today's Farallon Islands are, and the Juan de Fuca Plate was snipped off the north end of the Farallon plate forever.
The Pacific Plate and the North American Plate, rather than form another subduction zone, began to slide against each other along the transform fault, which we now call the San Andreas Fault. A fragment of North America (recently scraped-up sediments) was rifted off and welded to the Pacific plate, forming Baja California. Two triple junctions formed and moved away from each other; one is now at Cape Mendocino in California, and the other is just off Mazatlan, Mexico. Finally, around 23 million years ago, the remnant of the Farallon Plate split into the Cocos Plate subducting beneath Central America and the Nazca Plate subducting beneath South America.
Even buried beneath North America, the Farallon Plate is still not dead. Seismologists are able to model1 the disintegrating remnants of the plate several hundreds of kilometers beneath the crust, with the furthermost corner under the Gulf of St. Lawrence.
1The chewed-up bits of the Farallon plate embedded in Earth's mantle depicted here show a the results of Dr. Hans-Peter Bunge's theoretical model of Earth's mantle, TERRA.