Origin 800 BCE to 500 BCE
Karma means action, and by extension the consequence that action carries. The general principle is shared: what is done conditions what follows, across lives as well as within them. Beneath that the traditions differ enough that a single account of karma would misrepresent all of them.
It is not divine judgement. In the classical formulations nobody sits in judgement, no sentence is passed, and no record is kept by an official. The process is closer to a natural regularity than to a court, and it is impersonal in the way that a law of physics is impersonal. Some Hindu theistic schools do bring God in as the agency that dispenses the fruits of action, on the argument that unconscious residues cannot arrange their own consequences, but that is a specific theological addition made in the face of a recognised difficulty, not the default meaning of the word.
Buddhism makes karma a matter of intention. The Buddha's reported identification of action with volition is the pivot of the Buddhist account, and it moves the whole subject inward: an act done without intention does not carry the same weight, and the mental disposition rather than the outward event is what conditions the future.
Jainism is the outlier and the most concrete. Karma there is a subtle material substance which flows into the soul and adheres to it, obscuring its innate qualities in something like the way dust obscures a surface. This is not a metaphor within the system. It explains why Jain practice is organised around stopping the influx and shedding what has accumulated, and why Jain asceticism is more severe than its neighbours: if the problem is accreted matter, then austerity is a technique for removing it.
The popular English use, in which karma means an ironic comeuppance arriving promptly, has almost no relation to any of these.