In this screenshot, I want to know that the upside-down T is. (I'm not sure how to research it if I don't know its name) =)
(The context, is to prove that that $(S^{\perp})^{\perp}$ is the smallest closed subset of $H$ contained in $S$, where $S$ is a subspace of a Hilbert space $H$). But I just want to know the notation.
It refers to the orthogonal complement of a given subspace. So, $S^{\perp}$ is the orthogonal complement of $S$ and $(S^{\perp})^{\perp}$ is the orthogonal complement of $S^{\perp}$.