# What does it mean for something to be optically thick to pair production?

I have seen several references in the high energy astrophysics literature (including the example below) which make reference to an astronomical source being "thick to pair production".

What does this mean? Why would something be thick to pair production?

from Broderick and Tchekhovskoy (2015) Horizon-Scale Lepton Acceleration in Jets: Explaining the Compact Radio Emission in M87:

Given the threshold seed photon energy of 1.2 meV, the minimum gamma-ray energy for which the seed photons bath is optically-thick to pair production is $$\epsilon_{\gamma, min}$$ = 640 GeV, hence the asymptotic Lorentz factor for M87 is (see Equation 27)

$$\gamma_{\gamma \gamma} = 2.6 \times 10^6, \tag{59}$$

implying enhancement in the number density due to the postgap cascade of roughly $$n_{\infty} / n_g$$ = 670 (see Equation 28).

Mathematically this is often described in terms of an intensity traversing across a slab of material of thickness $$x$$ as $$I = I_0 \exp(-\rho \sigma x)\ ,$$ where $$I_0$$ is the intensity entering the slab, $$\rho$$ is the density and $$\sigma$$ is the absorption/scattering cross-section in units of area per mass.
If the product $$\rho \sigma x > 1$$ then little radiation emerges on the other side of the slab and it would be said to be "optically thick". Clearly a high density, a high cross-section or a large physical thickness, will make something optically thick.