What is 'karma' according to Buddhism?
Karma is a central and fundamental aspect of Buddhist doctrine common to all traditions of Buddhism. The Buddhist concept of Karma is a natural process that operates with all our volitional actions and which does not need an external agency or power that sits in judgement.

Karma in Buddhism should not be considered as moral justice, or a method of reward and punishment, that needs an external body or authority to decide whom or what should be punished and rewarded. It is believed that Gautama Buddha attained complete insight into how the process of Karma works during the night of his full enlightenment when, through the divine eye (dibbacakku), Buddha saw how other beings die and are reborn in happy and unhappy states according to their Karma.

According to the Cula-kammavibhanga sutta of the Majima Nikava (collection of the middle length discourses of the Buddha), a young brahmin by the name of Subha had approached the Buddha and inquired as to why there was so much inequality among human beings in terms of being short-lived or long-lived, sickly or healthy, ugly or beautiful, powerful or powerless, poor or rich, low-born or high-born, ignorant or intelligent, etc. Buddha had then explained that:
"all living beings are the owners of their actions, heirs to their actions, they originate from their actions, they are related through their actions, they have their actions as their refuge and that it is action or Karma that distinguishes beings as inferior and superior

The word Karma in Pali, and karma in Sanskrit, means action but encompasses only actions that are intentional, volitional and willful and which will lead to consequences sooner or later. The mind (citta) by itself cannot perform or direct any mental, verbal or physical action and it is the intention, will, mental effort or volition (cetana) that directs any mental, verbal or physical action.

Thus the Buddha declared:
"Cetanaham bhikkhave kammam vadami
Cetayitva kammam karoti, kayena, vacaya, manasa"

"It is volition that I call Karma, by volition one performs Karma through body, word or mind."