Mahābhūta is Pāli for the "Great Elements." The four Great Elements (Pali: cattāro mahābhūtāni) are earth, water, fire and air. Mahābhūta is generally synonymous with catudhātu, which is Pāli for the "Four Elements." In early Buddhism, the Four Elements are a basis for understanding and for liberating oneself from suffering.
The Four Elements are used in Buddhist texts to both elucidate the concept of suffering (dukkha) and as an object of meditation.
The Four Elements pertinence to the Buddhist notion of suffering comes about due to:
The Four Elements are the primary component of "form" (rūpa).
"Form" is first category of the "Five Aggregates" (khandhas).
The Five Aggregates are the ultimate basis for the "Four Noble Truths." 
Schematically, this can be represented in reverse order as:
Four Noble Truths → Suffering → Aggregates → Form → Four Elements
Thus, to deeply understand the Buddha's Four Noble Truths, it is beneficial to have an understanding of the Great Elements.
In the Mahasatipatthana Sutta, in listing various bodily meditation techniques, the Buddha instructs:
"...Just as if a skilled butcher or his assistant, having slaughtered a cow, were to sit at a crossroads with the carcass divided into portions, so a monk reviews this very body ... in terms of the elements: 'There are in this body the earth-element, the water-element, the fire-element, the air-element.' So he abides contemplating body as body internally...."(Walshe, 1995, p. 338.)
According to the Visuddhimagga, Ch. XI, para. 27, the Four Elements are briefly referenced in:
the Mahasatipatthana Sutta ("The Greater Discourse on the Foundations of Mindfulness," DN 22)
The Four Elements are described in detail in:
the Mahahatthipadompama Sutta ("The Greater Discourse on the Simile of the Elephant's Footprint," MN 28)
the Maharahulovada Sutta ("The Greater Discourse of Advice to Rahula," MN 62)
the Dhatuvibhanga Sutta ("The Exposition of the Elements," MN 140)
In addition, the Visuddhimagga XI.27ff (Buddhaghosa, 1999, pp. 343ff.) has an extensive discussion of the Four Elements.
Definition of the Four Elements
In the aforementioned suttas (MN 28, 62, 140), the Great Elements are described as follows:
Earth elements may be either external or internal. Internal earth elements include head hair, body hair, nails, teeth, skin, flesh, sinews, bone, organs, intestinal material, etc.
Water elements may be either external or internal. Internal water elements include bile, phlegm, pus, blood, sweat, fat, tears, nasal mucus, urine, etc.
Fire elements may be either external or internal. Internal fire elements include those bodily mechanisms that produce physical warmth, aging, digestion, etc.
Air elements may be either external or internal. Internal air elements includes air associated with the pulmonary system (for example, for breathing), the intestinal system ("winds in the belly and ... bowels"), etc.
According to the Visuddhimagga XIV.35 (Buddhaghosa, 1999, p. 443), "as to the proximate cause, each [element] has the other three as its proximate cause."
^ Or, more literally, "Great Natures." See Rhys Davids & Stede (1921-25), p. 507, entry for "Bhūta."
^ Note that the Pāli word dhātu is used in multiple contexts in the Pāli canon. For instance, Bodhi (2000), pp. 527-8, identifies four different ways that dhātu is used including in terms of the "eighteen elements" and, as in this article, in terms of "the four primary elements."
Bodhi, Bhikkhu (trans.) (2000). The Connected Discourses of the Buddha: A Translation of the Samyutta Nikaya. Boston: Wisdom Publications. ISBN 0861713311.
Buddhaghosa, Bhadantācariya (trans. from Pāli by Bhikkhu Ñāṇamoli) (1999). The Path of Purification: Visuddhimagga. Seattle, WA: BPS Pariyatti Editions. ISBN 1928706002.
Ñāṇamoli, Bhikkhu (trans.) & Bodhi, Bhikkhu (ed.) (2001). The Middle-Length Discourses of the Buddha: A Translation of the Majjhima Nikāya. Boston: Wisdom Publications. ISBN 086171072X.
Nyanaponika Thera (trans.) (1981). The Greater Discourse on the Elephant-Footprint Simile. Kandy, Sri Lanka: Buddhist Publication Society.
Rhys Davids, T.W. & William Stede (eds.) (1921-5). The Pali Text Society’s Pali–English Dictionary [PED]. Chipstead: Pali Text Society. A general on-line search engine for the PED is available from the University of Chicago's "Digital Dictionaries of South Asia" at http://dsal.uchicago.edu/dictionaries/pali/ (retrieved 2007-06-14).
Thanissaro Bhikkhu (trans.) (2003). Maha-hatthipadopama Sutta: The Great Elephant Footprint Simile. Available on-line at: http://www.accesstoinsight.org/tipitaka/mn/mn.028.than.html.
Walshe, Maurice O'C. (trans.) (1995). The Long Discourses of the Buddha: A Translation of the Digha Nikaya. Boston: Wisdom Publications. ISBN 0861711033.
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