I like to revisit this question every year because as education changes and evolves, so does the definition of instructional design. Postings like these are always unsatisfactory because it is difficult to say that there is one thing that we do. There are a lot of moving parts.
Traditionally, instructional design is the study, practice, and process of systematically designing and developing courses, instructional materials, and learning experiences in a consistent fashion towards the effective and engaging acquisition of knowledge. The process consists of working closely with faculty to determine the state and needs of the learners, defining the outcomes of instruction, and applying the appropriate pedagogy and “interventions” to create meaningful engagement in teaching and learning.
Instructional designers work with faculty to create learning materials and experiences in face-to-face, online, and hybrid classes. Notice that the above definition does not mention education technology, multimedia, or any particular tools, but instructional designers, depending on their abilities and background, will often have skills in those tools as a means for creating effective student engagement.
Instructional designers may also review courses and course content for functionality, inclusiveness, and accessibility; assist in the broader development of programs and curricula; assist faculty in the use or creation of open education resources, and provide professional development opportunities for faculty and staff.
I call instructional designers the “Swiss Army Knives” of education: we do a little bit of everything. Instructional designers, for instance, do a lot of work on campuses to promote social justice (often without knowing it) by promoting accessibility of course materials and teaching and learning environments, or through the promotion and advocacy of open education resources and open education practices.
Feel free to leave a comment below if you have a favorite definition of instructional designer or instructional design.