# What Is Force? Force is a derived description of reconfiguration and momentum transfer. When a bounded region changes its motion, what has changed physically is the flux of momentum through its boundary. "Force" is the compact name for that boundary accounting. This also clarifies the status of so-called force fields. If two supposedly independent substrates interact, they are not truly independent. A mediating field does not preserve the split. It exposes a deeper common structure in which the coupling is taking place. Force is descriptive. It is bookkeeping for coupled change within one underlying physical organization.