A Reduced-Order Model is a feature in a COMSOL Multiphysics model that encapsulates important aspects of the behavior of another model in a computationally efficient form. From the perspective of a COMSOL Multiphysics model that uses a reduced-order model, it is essentially a black box with a number of inputs and a number of outputs. Inside the box is an online model, which has been trained so as to reproduce the input-output behavior of a full, unreduced, COMSOL Multiphysics model and study step. Reduced-order models are normally created and trained by a Model Reduction study step (see Model Reduction) but can also be imported from a COMSOL Reduced-Order Model file (*.mphrom).

Outputs from a reduced-order model are made available to the model where it is used (the caller) in the form of variables or operators. These can be part of expressions and equations when computing the calling model, but can also be evaluated directly in postprocessing.

When a model contains a reduced-order model, it is represented as a node under the Reduced-Order Modeling node () under Global Definitions. See the following sections for information about the available reduced-order model types, concepts, and associated modeling tools: