A servitor is a mental construct that serves a purpose. The term is generally used to refer to any mental construct that is able to perform a function, while having no own sense of agency, or sometimes even interchangeably with "mental construct". A servitor is not self-aware, and one could not for instance hold an interesting conversation with one, unlike a tulpa. Despite this, it's possible to place considerable effort into creating a servitor fit for a particular purpose.
Servitors in practice
Servitors are often created as various mental automations. An example of a servitor would be a traditional imaginary friend; an entity, often a mind-form that responds to stimuli in certain learned patterns, but with no independent thought behind it. It's commonly accepted that servitors of such nature can be turned into tulpas, by allowing them a sense of agency over their own actions.
Another example of a servitor would be a mental routine that moves one's arm in a certain pattern, with no attention paid to it by the "host". The ability to drive a vehicle "automatically", could be considered a servitor.