One who handles something (especially manually) or someone.
An advisor or manager to a person occupying a position or office to which the speaker believes the holder does not possess the typical qualifications and/or experience.
A subroutine that handles a particular situation such as an event or exception.
A controller, trainer, someone who handles a specified thing, animal or person (especially a prizefighter).
An operative person or thing.
Any of a class of behaviors that produce consequences by operating (i.e., acting) upon the environment.
That operates to produce an effect.