π for 3.14159... Early writers indicated this constant as a ratio of two values. William Oughtred (1574-1660) designated the ratio by the fraction π over δ in Clavis mathematicae. The symbolism appears in the editions of this book of 1647, 1648, 1652, 1667, 1693, and 1694 (Cajori vol. 2, page 9).
φ for the golden ratio. According to The Curves of Life: Being an Account of Spiral Formations and Their Application to Growth in Nature, to Science, and to Art: With Special Reference to the Manuscripts of Leonardo da Vinci (1914) by Sir Theodore Andrea Cook (1867-1928), page 420:
Mr. Mark Barr . . . suggested . . . that this ratio should be called the phi proportion for reasons given below . . . The symbol phi was given to this proportion partly because it has a familiar sound to those who wrestle constantly with pi and partly because it is the 1st letter of the name of Pheidias, in whose sculpture this sculpture is seen to prevail when the distance between salient points are measured.
The above quotation and citation were provided by Samuel S. Kutler and Julio González Cabillón. Barr was an American mathematician.
According to Gardner (1961) and Huntley, the letter phi was chosen because it is the first letter in the name of Phidias who is believed to have used the golden proportion frequently in his sculpture. However, Schwartzman (page 164) implies the letter stands for Fibonacci.
The Greek letter tau is also used for this constant. Tau is found in 1948 in Regular Polytopes by Harold Scott MacDonald Coxeter, according to John Conway, who believes Coxeter may have used the symbol in his papers of the 1920s and 1930s. Ball and Coxeter (1987, page 57) write, "The symbol [tau] is appropriate because it is the initial of tomh\ ("section") [Antreas P. Hatzipolakis].
H. v. Baravalle used G for 0.618... in "The Geometry of the Pentagon and the Golden Section," which appeared in The Mathematics Teacher in January 1948. He may have used the same symbol in his "Die Geometrie des Pentagrammes und der Goldene Schnitt" in 1932.
In The Shape of the Great Pyramid (1999), Roger Herz-Fischler uses G for 1.618... and g for .618....