The golden ratio, denoted by the Greek letter phi (φ), is a mathematical constant approximately equal to 1.618. It is closely linked with the Fibonacci sequence, a series of numbers where each number is the sum of the two preceding ones. Starting with 0 and 1, the sequence progresses as 0, 1, 1, 2, 3, 5, 8, 13, 21, and so on, with the ratio of each number to its predecessor converging towards 1.618, or phi. This principle can be applied to UI spacing and is adaptable for various devices and screen sizes.