Malapropism is the unintentional use of a word that sounds similar to the intended word but has a completely different meaning, often resulting in a humorous effect. This occurs when someone confuses one word with another that sounds alike, leading to a mix-up in communication. For example, saying "He is the pineapple of politeness" instead of "He is the pinnacle of politeness" illustrates a malapropism. Such mistakes can add humor to conversation but may also cause confusion if the listener does not understand the intended meaning.