This tier of vocabulary contains content-specific vocabulary, often defined in textbooks. What is it referred to as?

The correct answer is Tier 3 Vocabulary. This tier consists of specialized, domain-specific terminology that is often found in textbooks and is relevant to particular subjects or fields of study. For example, in a science textbook, words like "photosynthesis" or in a social studies textbook, terms such as "democracy" illustrate this specialized vocabulary. Learners encounter Tier 3 vocabulary in specific contexts and usually need direct instruction to understand and use these terms effectively within their respective disciplines.