Understanding the definition of polynomially CPA (IND-CPA) security I just started on encryption schemes and have some trouble in understanding the following definition of IND-CPA security:

$E$ is polynomially CPA (IND-CPA) secure, if for all adversaries $A$ and all polynomials $p$, we have: $Adv_a &lt;= \frac{1}{p(\lambda)}$ for sufficiently large $\lambda$, i.e.: For all $A$ and all $p$, there exists an $N_p&gt;0$ such that $Adv_a \leq \frac{1}{p(\lambda)}$ for all $\lambda &gt; N_p$.

As far as I understand, in a simple IND-CPA game, an oracle sends a ciphertext to an adversary that is the encryption of one of two possible messages chosen by the adversary. An adversary now needs reply with what the actual message is. 

What I don't understand is how I should interprate the polynomials $p$ and parameters $\lambda$. What do they consist of?
As explained on Wikipedia:


  The reciprocal-of-polynomial formulation is used for the same reason that computational boundedness is defined as polynomial running time: it has mathematical closure properties that make it tractable in the asymptotic setting. For example, if an attack succeeds in violating a security condition only with negligible probability, and the attack is repeated a polynomial number of times, the success probability of the overall attack still remains negligible.


Which is why you want it to work for all polynomial $p$ in your definition.

It is also important to notice that in this definition, it is implied that the adversary has a polynomially bounded computational power. 

Which means basically that in the IND-CPA game, the adversary has the right to perform a polynomially bounded number of encryptions or other operations, before sending the two plaintexts to the challenger, which will choose one and return its encryption to the adversary, who must now guess to which plaintexts it corresponds.

Let me develop this with an example:
in the RSA-OAEP case, this forbids for instance the adversary to factor the public key using the GNFS since it has a complexity of :
$$\exp\left( \left(\sqrt[3]{\frac{64}{9}} + o(1)\right)(\ln n)^{\frac{1}{3}}(\ln \ln n)^{\frac{2}{3}}\right) =L_n\left[\frac{1}{3},\sqrt[3]{\frac{64}{9}}\right]$$
for factoring the number $n$, but we're usually interested by the security parameter, which, in RSA-OAEP case, is the $\lambda=\log_2(n)$ , but this is not a problem, since its running time is super-polynomial but sub-exponential in the size $\lambda$ of the input $n$:
$$O\left(\exp\sqrt[3]{\frac{64}{9} \lambda (\log \lambda)^2}\right)$$ so it is still not a polynomial algorithm and thus cannot be considered when studying the IND-CPA security of RSA-OAEP.

However, if factorization were to be proven polynomial, then the adversary would win the IND-CPA game in the RSA-OAEP case thanks to it.  

PS: As mentioned below, plain RSA is not IND-CPA since it is deterministic.