Semantic Externalism is a philosophical theory that suggests the meaning of a word or expression is not solely determined by the mental states of the speaker. Instead, it posits that the context and external environment play a crucial role in shaping meaning. This means that two people can have different meanings for the same term based on their experiences and surroundings. For example, the term water refers to H2O, but its meaning can vary depending on the speaker's environment. If someone is in a different context, like a fictional world, their understanding of water might differ. Thus, meaning is influenced by external factors beyond individual thought.