As an activity, a favor or favour is a deed in which help is voluntarily provided.
It may be defined as something done or granted out of goodwill, rather than from justice or payment.[1] Yet, reciprocity (responding to a positive action with another positive action) may be a common motivation, but other motivations, e.g. altruism may rather be the main ones where the action isn't to return the favor but to pay it forward.
The will to make up for a favor may largely be independent of whether the original favorer would actually get to know of the compensation, indicated by a study where participants were unexpectedly done a favor and later asked by the favorer to fill in and return a survey, and whether the favorer would receive it personally or if it would be left in adrop-off box didn't significantly alter the amount of people returning the survey.[2]
It may be used as an alternative to monetary compensations in the form of currency, or other items of value. A drawback compared to objects is that the value of a favor is hard to estimate, and depends on the situation and the person providing it.
In casual sense, it is generally what is referred to as one in the expression I owe you one.