The study of secondary mathematics involves many ideas, but too often high school mathematics curriculum materials and instruction begin by giving students the vocabulary first, along with definitions, before students have been able to explore the ideas. Particularly for students bureaucratically designated as English Learners, preteaching definitions can actually hinder their understanding rather than enabling them to quickly and deeply explore important mathematical ideas (e.g., de Araujo & Smith, 2022). In the interest of developing a more productive approach, this brief considers how definitions are currently introduced and used in mathematics education at the secondary level. The brief addresses the following questions with the aim of helping educators ensure that English Learners can participate fully in mathematical learning: (1) What challenges do teachers need to consider for using good mathematical definitions with English Learners?; (2) How do curriculum materials currently approach the definition of key terms?; and (3) How can learning opportunities focus on ideas that enable English Learners to explore and generate their own definitions?