"In all perfectly beautiful objects
there is found the opposition of
one part to another and
a reciprocal balance"

John Ruskin,

art critic

Altough it deals with such a simple and complicated concept at the same time, as to make a variety of definitions arise about it, all them turn around the most usual mode to outline the Golden Section, syn­the­sized eg. by the
Merriam-Webster's Definition of golden section
: a proportion (such as one involving a line divided into two segments or the length and width of a rectangle and their sum) in which the ratio of the whole to the larger part is the same as the ratio of the larger part to the smaller"
Widely known also as the Divine Proportion, it’s the only number that can multiply or divide others, keeping always - like a sort of hinge - the same proportion among the resulting portions.

It deals with a unequaled recurrent relation among “media and ex­treme ratio”, perpetually equal to itself in opposite directions, which virtually brings the concept of simultaneity of the move­ment both towards the future j) and towards the past j).
Indeed it owns a double, irrational face; so if this is Phi:

F = 0.61803398874989484820458683436563811772030917980576

this is phi:

j = 1.61803398874989484820458683436563811772030917980576