Definition
We say that a closed subset \(D\subset X\) is normal crossings if for all \(x\in D\) there is an affine neighborhood \(U\) with coordinates \(x_1,...,x_n\) such that \(D\cap U\) is given by \(x_1\cdot...\cdot x_p = 0\) for some \(p\leq n\).