Equal sign with a “c”

Question

I am not sure whether this is common sense, but I really don't know what this $c$ means in an equivalent sign, from Lindell's How to Simulate It:

$$\big\{(S_1(1^n, x, f_1(x,y)), f(x,y))\big\}\stackrel{c}{\equiv} \big\{(\mathsf{view}^\pi_1(x,y,n),\mathsf{output}^\pi(x,y,n))\big\}_{x,y,n}$$ $$\big\{(S_2(1^n, x, f_2(x,y)), f(x,y))\big\}\stackrel{c}{\equiv} \big\{(\mathsf{view}^\pi_2(x,y,n),\mathsf{output}^\pi(x,y,n))\big\}_{x,y,n}$$

Answer 1

To quote the paper

Two probability ensembles... are said to be computationally indistinguishable, denoted $X\stackrel{c}{\equiv}Y$, if...

This is found in section 2.