What is meant by $\tilde\Omega(\lambda^4)$? I'm currently reading the paper (Leveled) fully homomorphic encryption without bootstrapping , and the following paragraph was near the start:   



What is meant by the symbol used? Is it merely to represent polynomial time or something else entirely?
As you probably know $f(\lambda)=O(\lambda^4)$ means that $|f|$ asymptotically upper bounded by some constant times $\lambda^4$. The notation $f(\lambda)=\Omega(\lambda^4)$ corresponds to an asymptotic lower-bound.

Now, the $\tilde O$ and $\tilde \Omega$ are closely related notations, where we not only ignore constants but also values which are polynomial in the logarithm of the argument.

So in your quotation, $\tilde\Omega(\lambda^4)$ refers to a function $f(\lambda)$ which is asymptotically lower bounded by something of the form $K \log(\lambda)^c \lambda^4$, where $K$ and $c$ are some non-specified constants. [I am ignoring the absolute value of $f$ from the definition, since it clear in your context that $f$ takes positive values.]