A Greek letter commonly used to represent functions, angles, or scalar fields in mathematics and physics.
Examples
Angle in geometry or trigonometry
\sin(\varphi) = \frac{\text{opposite}}{\text{hypotenuse}}
Golden ratio definition
\varphi = \frac{1 + \sqrt{5}}{2} \approx 1.618034
Euler's totient function
\varphi(n) = \#\{k \in \mathbb{N} : 1 \leq k \leq n, \gcd(k,n) = 1\}