Dominant rational map

Definition
Let $$X$$ and $$Y$$ be varieties over a field $$k$$. A rational map from $$X$$ to $$Y$$ is said to be dominant if for some (and hence for every) pair $$(U,\phi_U)$$ representing the rational map, the image of $$U$$ under $$\phi_U$$ is dense in $$Y$$.