The key formally known as 'Prince'

An "extended" Macintosh keyboard—the most common type—has two keys formally known as 'Prince', one on each side of the space bar. Some compact keyboards have one only on the left.

So named after the musician "Prince" who changed his name to a symbol (than back to "Prince")

Also known as "The Command Key", "open-Apple key", "Apple key", "Apple modifier key"