Phi (Φ and φ, pronounced [fī]) is a Greek letter. In mathematics it is used to represent the number 1.6180339887..., the key number in the Golden Ratio. Many living organisms grow according to the golden ratio; the bones in a human finger are proportioned in a ratio of approximately 1:1.6; the branching of veins and nerves in animals and humans follow similar proportions. Phi is an essential number when calculating the dimensions of a Fibonacci Spiral.
The letter phi appears on the Frog Glyph. Fibonacci Spirals and the number 1.6180339887... also appear alongside other glyphs.
In Walter's Lab Notes, Walter refers to the shape of the room containing the laboratory without specifying what the shape is.