A relational symbol representing a thicker version of "approximately equal to" used for stronger approximations in analysis.
Examples
Approximate equivalence between functions
f(x) \thickapprox g(x) \text{ for large } x
Statistical approximation with stronger similarity
\hat{\theta}_n \thickapprox N(\mu, \sigma^2)
Asymptotic behavior
\ln(n!) \thickapprox n\ln(n) - n \text{ as } n \to \infty