Establishing a common language in research is essential but challenging in the context of gender/sex entanglement. Without agreement and understanding of specific terms, study design and analysis of findings risk bias, misinterpretation, or misrepresentation. This chapter scrutinizes working definitions and argues that animal research can incorporate the concept of gender by modeling components of human gendered expectations or interactions in ethological contexts relevant to these expectations and interactions for animals. It looks at the considerable variation in physiological and social factors that contribute to sex and gender, and analyzes gender/ sex entanglement across multiple levels of biological organization and different life stages. Particular attention is given to critical periods of brain development. It advocates for incorporating gender/sex entanglement into the conception, conduct, and communication of science, and discusses new initiatives and resources that can support studies of gender/sex entanglement. Illuminating how deeply sex and gender are entangled—and all the attendant complexity—holds promise for expanding the acceptance of that complexity in all corners of society.