Undergrad Symbol meaning in CP-violating angle

[Ranku]

I saw this symbol in the statement: 'Effective CP-violating angle θ≅

(10^-17). What does the highlighted symbol mean?

 

[vanhees71]

This is a Landau symbol. A quantity is called of order $f(x)=\mathcal{O}(x^{n})$ if
$$\lim_{x \rightarrow 0} \frac{f(x)}{x^n}=C=\text{const}.$$
In this somewhat abuse of notation it means that $\theta/10^{-17}$ is a value around 1.