The process of rapidly learning a new word by contrasting it with a familiar word. This is an important tool that children use during language acquisition.
An example would be presenting a young child with two toy animals - one a familiar creature (a dog) and one unfamiliar (a platypus). When the child is asked to retrieve the platypus a contrast is provided for the child (dog versus unknown creature) which allows them to infer the other creature must be a platypus. Using fast mapping is very effective in new word acquisition with most children being able to recall the new object a week later.