In linguistics, coercion is a term applied to a process of reinterpretation that is triggered by a mismatch between the semantic properties of a selector and the semantic properties of the selected element. The term was first used in the semantic literature by Marc Moens and Mark Steedman, who adopted it on "loose analogy with type-coercion in programming languages".
In the sentence "I began the book", the predicate "began" is assumed to be a selector which requires its complement to denote an event, but "a book" denotes an entity, not an event. So, on the coercion analysis, "begin" coerces "a book" from an entity to an event involving that entity, allowing the sentence to be interpreted to mean, e.g., "I began to read a book," or "I began to write a book."
Coercion is closely related to the notions of active zone, construal/conceptualization, and syntactic accommodation known from various schools within the cognitive linguistics movement.