In mathematics, particularly in set theory, a model is some concrete realization of a set of axioms (in a sense this is the opposite of the previous definition). See model theory.
In Engineering, a solid model may refer to 3D CAD representation, see solid modelling.
A scale model is a replica or prototype of an object built either for research or as a hobby, usually built smaller than the existing or intended thing, though can equally be built larger to illustrate something that would otherwise be hard to see.