Background: Instructional differentiation within classrooms involves educators' adaptations of instructional elements to meet diverse learners' needs. It is usually regarded as a crucial component of high-quality education that promotes equal and inclusive opportunities for all students. However, defining and operationalising this complex construct is challenging, and important, in efforts to better understand instructional differentiation and support learners worldwide. Purpose: This paper aims to contribute to a better understanding of instructional differentiation. It discusses definitions and operationalisations of instructional differentiation in the educational research literature and argues for the inclusion of "deliberateness" and "adaptiveness" as two defining characteristics of instructional differentiation. Sources of evidence and main arguments: Using theoretical arguments and illustrations from empirical research, including a small-scale study of our own, we discuss and exemplify the value of considering deliberateness and adaptiveness in empirical research on instructional differentiation. Further, we consider the challenges and opportunities for research on instructional differentiation. Conclusions: Studying the deliberateness and adaptiveness of instructional variation calls for conceptual and operational alignment, and research methodologies that take into account the multiple perspectives of teachers and students. Our investigations draw attention to the conceptual complexity of instructional differentiation, challenges in practice and the need for professional development to support teachers' embedding of instructional differentiation practices.