Within the field of cache-oblivious algorithms the ideal cache model is used for determining the cache complexity of an algorithm. One of the assumptions of the ideal cache model is that it models a "tall cache". This is given by the statement $Z = \Omega(L^2)$. Where $Z$ is the size of the cache and $L$ is the size of the cache line. What does $\Omega$ represent?
It's the lower bound counterpart to O($\cdot$). Z is larger than some constant times $L^2$
It's called "asymptotic lower bound" - https://www.khanacademy.org/computing/computer-science/algorithms/asymptotic-notation/a/big-big-omega-notation