I am reading Birkenhake-Lange's Complex Abelian Varieties. In Chapter 3 section 1, there is a notion of isotropic for lattices:
Let $X=V/\Lambda$ be a complex abelian variety, where $V$ is a complex vector space and $\Lambda$ is a lattice. Let $L$ be a line bundle on $X$ with first Chern class $H$ (which is a hermitian form) and $E=\text{Im }H$ be its alternating form.
A direct sum decomposition $$\Lambda=\Lambda_1 \oplus \Lambda_2$$ is called a decomposition for $L$ if $\Lambda_1$ and $\Lambda_2$ are isotropic with respect to the alternating form $E$.
I am not sure what this "isotropic" means, I also can not find any other books/papers using this word under this context. I guess maybe it is saying that the form vanishes along lattice points?
Any comment is appreciated.