φ is the symbol for the golden ratio.
Two quantities a and b are said to be in the golden ratio φ if:
a + b = a = φ
Spirals are great examples of the presence of φ in nature: "a golden spiral is a logarithmic spiral whose growth factor is φ . A golden spiral gets wider (or further from its origin) by a factor of φ for every quarter turn it makes.
It has been called the hand of God in nature, and artists of all times have tried to respect it intuitively or consciously...