A Greek letter commonly used to represent characteristic functions, random variables, or chi-square distributions in statistics.
Examples
Character/characteristic function notation
\chi_A(x) = \begin{cases} 1 & \text{if } x \in A \\ 0 & \text{if } x \notin A \end{cases}
Chi-squared statistic in statistics
\chi^2 = \sum_{i=1}^n \frac{(O_i - E_i)^2}{E_i}
Euler characteristic in topology
\chi(M) = V - E + F