In general physics, a force field is a vector field representing the gradient of a potential. The vectors that are the values of a force field are forces, and so measured in units of force such as newtons and poundss.
In the context of molecular dynamics, a force field is a more loosely defined term and refers to the functional form and parameter sets used to describe the interactions (potential, forces) within a system of particles (atoms or similarly sized objects). It is independent of the system's configuration, and is not a numerical field as in the above context. A force field can be empirical, derived from higher-level modelling (e.g. quantum chemical studies) or even heuristic.