In The Host, a host is a human body that has been unwillingly implanted with an alien parasite. But the book is less about the host, Melanie, than it is about the alien, Wanderer, because Wanderer finds that her human host if more powerful than any host she's had before. It ends up ruling her, governing her actions and overwhelming her emotions.
In the end, Wanda is defined by the body she is in. When she gets transferred into a new body at the end, we learn that the host will always define her.