What is instructional design?
Instructional design can be defined as the creation of instructional materials, modules or lessons.
The instructional design process consists of determining the needs of the learners, defining the end goals and objectives of instruction, culminating assessment tasks, and designing teaching and learning activities o ensure the quality of instruction.
Following an instructional design (e.g., ADDIE) model can help instructors to create meaningful and effective instruction.
Instructional design is a technology for the development of learning experiences and environments which promote the acquisition of specific knowledge and skill by students.
Instructional design is a technology which incorporates known and verified learning strategies into instructional experiences which make the acquisition of knowledge and skill more efficient, effective, and appealing. 
Instructional Design is the art and science of creating an instructional environment and materials that will bring the learner from the state of not being able to accomplish certain tasks to the state of being able to accomplish those tasks. Instructional Design is based on theoretical and practical research in the areas of cognition, educational psychology, and problem solving.
Sara McNeil defines the Instructional design as a:
Process: Instructional Design is the systematic development of instructional specifications using learning and instructional theory to ensure the quality of instruction. It is the entire process of analysis of learning needs and goals and the development of a delivery system to meet those needs. It includes development of instructional materials and activities; and tryout and evaluation of all instruction and learner activities.
Discipline: Instructional Design is that branch of knowledge concerned with research and theory about instructional strategies and the process for developing and implementing those strategies.
Science: Instructional Design is the science of creating detailed specifications for the development, implementation, evaluation, and maintenance of situations that facilitate the learning of both large and small units of subject matter at all levels of complexity.
Reality: Instructional Design can start at any point in the design process. Often a glimmer of an idea is developed to give the core of an instruction situation. By the time the entire process is done the designer looks back and she or he checks to see that all parts of the “science” have been taken into account. Then the entire process is written up as if it occurred in a systematic fashion.
The systematic process of translating a plan of instruction into a set of activities, materials, information and/or assessment procedures. 
A systematic process that is employed to develop education and training programs in a consistent and reliable fashion. 
The term instructional design refers to the systematic and reflective process of translating the principles of learning and instruction into plans for instructional materials, activities, information resources, and evaluation. 
The instructional design process incorporates the theoretical considerations presented above into a practice-based framework that bridges the gaps between learner needs, learning objectives, delivery of instruction, and evaluation. 
(A) systematic process, based on educational theories, on the development of instructional strategies, and specifications to promote quality learning experience. 
The development of instruction for specified goals and objectives in which (1) the organized sequential selection of components is made on the basis of information, data, and theoretical principles at every stage and (2) the product is tested in real-world situations both during development and at the end of the development process. 
- Merrill, M. D.; Drake, L.; Lacy, M. J.; Pratt, J. (1996). “Reclaiming instructional design” (PDF). Educational Technology. 36 (5): 5–7.
- Siemens, G. (2002). Instructional design in elearning. Retrieved January, 21, 2013. http://www.elearnspace.org/Articles/InstructionalDesign.htm
- Sara McNeil, http://www.coe.uh.edu/courses/cuin6373/whatisid.html
- Smith, P. L., & Ragan, T. J. (2005). Instructional Design (Third ed.). Hoboken, NJ: John Wiley & Sons, Inc.
- Reiser, R.A., Dempsey, J.V. (2007). Trends and Issues in Instructional Design (2nd ed.). Upper Saddle River, NJ: Pearson Education, Inc.
- Smith, P. L., & Ragan, T. J. (1999). Instructional design (2nd ed.). Upper Saddle River, NJ: Prentice Hall.
- Miner, K. R., Childers, W. K., Alperin, M., Cioffi, J., & Hunt, N. (2005). The MACH Model: from competencies to instruction and performance of the public health workforce. Public Health Reports, 120(1_suppl), 9-15.
- Mustaro, P. N., Silveira, I. F., Omar, N., & Stump, S. M. D. (2007). Structure of storyboard for interactive learning objects development. Learning objects and instructional design, 253-280.
- M.E.Gredler,LearningandInstruction:TheoryintoPractice,Fourth Ed. Merrill Prentice-Hall: Columbus, OH, 2001.