What process involves checking each word in a program to verify its validity within the language?

The process of checking each word in a program to verify its validity within the language is known as lexical analysis. This phase occurs during the compilation process and focuses on breaking down the code into tokens, which are the smallest units of meaning, such as keywords, identifiers, literals, and symbols. Lexical analysis serves to ensure that the sequences of characters in the code are valid according to the rules of the programming language, identifying any invalid tokens that may cause issues later in the compilation process.

This stage is crucial because it simplifies the structure of the code by converting it into tokens that the compiler can easily process in subsequent stages, such as syntax analysis. Syntax checking is a different process that involves checking whether the arrangement of these tokens follows the grammatical rules of the language, ensuring that the structure of the sentences is correct. Meanwhile, code review refers to a manual examination of code by developers to catch errors and improve quality, but it does not specifically address the validity of individual words in the programming context like lexical analysis does. Thus, lexical analysis is the correct answer as it directly relates to validating each word in programming terms.