The N400 is an event-related potential (ERP) component associated with the brain’s processing of meaning and semantic information. It is a negative-going electrical response that typically reaches its maximum amplitude around 400 milliseconds after a stimulus is presented, which is where its name comes from. The N400 was first identified by Marta Kutas and Steven Hillyard in 1980 while studying ERPs elicited by words appearing in sentences. Kutas and Hillyard discovered that the N400 became particularly large when a word was semantically unexpected given the preceding sentence. For example, in a sentence such as “The boy spread the warm bread with butter,” the word butter fits naturally with the context and produces a relatively small N400. Replacing it with an unrelated word such as “socks” produces a substantially larger negative-going response. Importantly, the effect was not simply a response to a visually unusual word. Their experiments found that semantic violations produced the N400, whereas physical abnormalities elicited a different, late positive response. Subsequent research showed that the N400 is more accurately understood as an index of semantic processing and contextual integration rather than simply a response to grammatical or semantic violations. Its amplitude varies continuously with the relationship between a stimulus and its preceding context. Words that are highly predictable from context tend to produce smaller N400s, whereas words that are less expected or less semantically related tend to produce larger ones. Kutas and Hillyard demonstrated this relationship using cloze probability, finding that N400 amplitude was inversely related to how strongly participants expected a particular word to complete a sentence. The N400 is therefore useful because it provides a millisecond-level measure of how the brain responds to meaning. It has been observed not only for written words, but also for spoken and signed language, pictures, objects, actions, faces, sounds, and even mathematical symbols. This broader evidence suggests that the N400 reflects a general aspect of accessing and integrating meaningful information rather than a mechanism dedicated exclusively to reading. One important point is that a larger N400 does not simply mean that a person “does not understand” a word. Instead, N400 amplitude is influenced by factors such as semantic association, contextual predictability, priming, and the availability of relevant information in memory. Consequently, researchers use the N400 as a window into how the brain dynamically retrieves and integrates meaning as information unfolds over time.