A representation of something else using principles that allow you to map properties of the model onto theoretically-selected properties of that something. By their very nature, all models are selective, and the selection (the ‘target system’ is determined by the purpose of the modeller and the materials available. Whatever they represent is important and they play a central role in scientific practice, examples being Bohr model of the atom and the double helix model of DNA. Model representations can depict their target system (or selected part of it) in a variety of different ways: scale models (e.g., bridges), analogical models (e.g., computer model of the mind), and idealized models (simplifying something intrinsically complex; e.g., classical mechanics model of the planetary system). Then there are models that are just be designed to provide tools for manipulating or intervening in the world. As phenomenological models, they are serve to represent properties of their targets while avoiding the postulation of hypothetical mechanisms.
See Analogy (as a trope), Collective agency, DNA double helix, Model, Model systems