Final answer:
Grammar is the complex set of rules that govern the formation of morphemes into meaningful words and sentences, including syntax and semantics, within a language's lexicon.
Step-by-step explanation:
The highly complex set of rules that regulate the formation of morphemes into words that convey meaning is called grammar. This set of rules incorporates both syntax and semantics. Syntax refers to the way in which words are organized into sentences, while semantics is concerned with the meaning derived from words and morphemes. The lexicon is a language's vocabulary, and grammar provides the structure to use this lexicon effectively to communicate a wide range of ideas and concepts. Children naturally learn the rules of language, indicating a biological predisposition within humans for language acquisition.