# phi

/ˈfaɪ/ (US) · /fˈaɪ/ (UK)

Source: https://englishreference.com/word/phi
Updated: 2026-08-15

## Senses

### 1. Greek letter

noun · CEFR B2

the 21st letter of the Greek alphabet.

For teachers: the twenty-first letter of the Greek alphabet, represented as Φ or φ.

- The professor used the symbol phi to represent a specific angle in the diagram.
- In classical Greek studies, the letter phi is often transliterated as 'ph' to distinguish it from the simpler 'p' sound.

### 2. the golden ratio

noun · CEFR C1 · Technical

a special number in math (about 1.618) that people often find beautiful in art and nature.

For teachers: an irrational mathematical constant, approximately 1.618, representing the golden ratio.

- Architects sometimes use phi to create buildings that look perfectly balanced.
- The Fibonacci sequence is closely related to phi, as the ratio between successive numbers approaches this value as the sequence progresses.

### 4. illusion of motion

noun · CEFR C2 · Technical

a trick for your eyes where flashing lights look like they are moving.

For teachers: the optical illusion of perceiving continuous motion between separate objects viewed rapidly in succession.

- The phi phenomenon is why a series of flashing neon lights looks like a moving arrow.
- Psychologists study the phi phenomenon to understand how the human brain synthesizes discrete visual inputs into a coherent sense of movement.

## Examples

- The symbol phi represents the golden ratio.
- Engineers use phi to calculate the stress on the bridge.
- In classical physics, phi often denotes the phase angle in wave equations.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.