Verifier - Complexity Theory A Verifier for a language $A$ is an algorithm $V$ such that $$A=\left\{ w \space | V  \space \text{accepts} \space \langle w,c\rangle \space\text{for}\space \text{some} \space \text{string} \space c\right\}.$$


My question about this definition is what is this string $c$ stands for? 

Let's say I have a graph and I want to verify if it has a clique. What would be my $c$?
A verifier for a language $L$ is an algorithm that accepts as input an instance $x$ of $A$ and a witness $w$, and has the following properties:


The algorithm always halts.
If $x \in A$ then there is a witness $w$ such that the algorithm accepts $\langle x,w \rangle$.
If $x \notin A$ then for all witnesses $w$, the algorithm rejects $\langle x,w \rangle$.


We are often interested in polynomial time verifiers. Such a verifier exists for the clique problem, which is: given a graph $G$ and a parameter $k$, decide whether $G$ contains a $k$-clique. Here is one polynomial time verifier for this problem: it accepts as witness a list $S$ of vertices, and on input $G,k,S$ it accepts if $S$ is a set of $k$ vertices which form a clique in $G$. I'll let you check that this satisfies all the required properties.