The adjective contrastive means "showing the difference between two things when you compare them" — like a contrastive analysis of American and British English.
To contrast two things is to think about how they are different. So, the adjective contrastive is used to describe things that differ from one another. Contrastive colors in a painting can create an interesting image, while contrasting personalities can spark a feud. In language, contrastive words show contrast between two parts of a sentence; words like but and though are examples.