Dhamma is that which bears (dhāraṇa) unique characteristics. It is not the thing itself. It is not its characteristics. It is the bearer of information about the thing and its characteristics. It is that which bears its nature, that, as S. N. Goenka said,  “Which is imbibed (dharetῑti dhammam), that which is lived by, that which upholds the particular characteristic essence of a thing”. Without “that which upholds” the thing cannot stand.

Dhamma is “that which has the mark of bearing its own nature” (sabhāva–dhāraṇo – bearing its own becoming), i.e. that which is not dependent on any more ultimate nature, the building brick of the nature.

The dhammas are conditioned and unconditioned.

Conditioned dhammas are so called, because they depend on causes and conditions. All conditioned dhammas are impure. Why is that? They are impure because the defilements adhere to them.

What are the unconditioned dhammas? Nirvāṇa (Pāli: Nibbāna) is unconditioned and pure, because the defilements do not adhere to it.

All dhammas – conditioned and unconditioned – are classified as:

(1) paramattha dhammas (supreme, ultimate dhammas),

(2) nāma–rūpa (mind-and-matter or name-and-form), and

(3) skandhas or aggregates; the five skandhas of attachment.

Paramattha dhammas are: mind, mental states, matter and Nibbāna.

The first three paramattha dhammas – mind (citta), mental states (cetasika), and matter (rūpa) – are also called nāma–rūpa, mind-and-matter or name-and-form. They all are conditioned and are explained in detail here.

The first three (physical mind, and mental states) are also classified as the fivefold skandhas (Pali: khandha) or aggregates to which one clings. They are: 1) physical matter, rūpa; 2) feeling, vedanā, 3) perception, saññā, 4) mental constructions, sankhāra, and 5) consciousness, viññāna.

Whatever they are called, paramattha (ultimate), nāma–rūpa or five khandhas, all conditioned dhammas are the world, arising, suffering, the locus of false views and opinions. They are the very existence. And the process of this very existence is occurrence.

Dhamma is also translated as phenomenon, state, appearance, thing, object of mind (dhamma–āyatana) – past, present or future, physical or mental, conditioned or unconditioned, real or imaginary. It could be likened to a mental event or mental state lasting one Planck’s moment or 10^-44 sec.

In the Preface to Dhātukathā Ven. Thein Nyun said: “In the wink of an eye or a flash of lightning, which lasts for a microsecond (10^–6 sec), at that instance, the mental elements or thought impulses arise and cease a trillion (10¹⁵) times. Thus the mental elements or thought-impulses arise and cease 10^-15 x 10^–6 = 10^-21 times per second.”

This is just an estimate.

Ven. Bhikkhu Samāhita, gives even a higher figure of 10^-44

All conditioned dhammas are so called because they depend on conditions. Conditioned, saṁskṛta, (Pali: saṇkhata) is “that which has been created”. In fact, a dhamma does not change its nature by changing its time period (past, present or future). The apparent change and the illusory movement of a dhamma is its serial continuity – a series of successive moments, a series of different momentary states in adjacent locations.

There is no dharma which is engendered by a single cause. Besides, it exists after having been non-existent. How is that?

(1) Each moment (dhamma, or mental impulse) exists after having been non-existent (time is discrete, not continuous; time is not a continuum, it is a succession of moments): its existence, following upon its non-existence, is its arising.

(2) After having existed, it does not exist anymore: this is its disappearance, death. When it disappears, it goes nowhere, meaning that the dhamma dies just where it arose.

(3) The duration of the dhamma is the concatenation or the process of successive moments: in fact, if the subsequent moment resembles the previous moment, it is then its substitute: then we say that the previous dhamma still exists or still lasts. Thus the subsequent moment (dhamma) can be considered as the duration of the previous moment.

(4) The dissimilitude of duration is its transformation.

All conditioned dhammas arise because of causes and conditions, meaning no dhamma arises by chance, at random, or just like that in and by itself. Given all proper conditions, any conditioned dhamma, any conditioned phenomenon or appearance will arise. No one can ever prevent it from arising and no god has anything to do with its creation. Given all proper conditions any dhamma will arise.

Only Nibbana is absolutely conditions free! It does not arise, it does not perish, it does not die. That is why Nibbana is called the “deathless element”, the “form” of happiness that does not disappear, the absolute peace and non-arising!

The conditions themselves are different dhammas and in turn they become conditions for other dhammas to arise. This dependency incessantly produces new states, new phenomena, new appearances and this is what Buddha called Dependent Origination. It also is the constant play of Samsara.

There are only appearances, dhammas in various combinations, resulting from our intentional actions, following the law of kamma (Skt: karma): action-result; cause and effect.

Being neither form nor mind, all dhammas, including Nibbana, are empty of “here and now”, empty of soul, empty of essence, empty of everlasting substance of any kind. All dhammas, including Nibbana, are rooted in Emptiness – this is the Teaching of the Buddha. This is what he calls “Ultimate Reality” or “Suchness”, tathatā.

Not seeing the empty nature of all appearances, not seeing cause and effect, not seeing impermanence, not seeing that there is neither divine particle, nor any permanent essence whatsoever, regardless the efforts, regardless the practice of severe restrictions and deprivations, one cannot attain Nibbana. Hence, understanding Emptiness of all dhammas is crucial for deliverance.

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As the teaching of the Buddha is the most meticulous classification of all the dhammas and the causal relations between them, it is highly recommended to study this classification here.