What is "emptiness meditation"?

Ven. Nyanatiloka, Buddhist Dictionary: A Manual of Buddhist Terms and Doctrines, edited and expanded by Dhr. Seven, Amber Larson, Bhante, Ashley Wells, Wisdom Quarterly

"Circle...O Zen empty spot, there is no thing you are and nothing you are not."

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The enso in Zen Buddhism suggestd emptiness.

What is "emptiness meditation"? It is suññatānupassanā: the "contemplation of emptiness," that is, the systematic analysis of not-self for the winning of liberating-insight.

It is one of the 18 chief kinds of insight or vipassanā meditations (cf. The Path of Purification, Vis.M. XXI). 

Suñña (adj.), suññatā (noun), in Sanskrit shunyata, is the verifiable fact that all things are void, empty, devoid of "self."
 

As a doctrinal term it refers in Theravāda Buddhism exclusively to the doctrine of no-self or egolessness (anattā), that is, the universal characteristic that all things are impersonal.

The phenomena that constitute all things (in particular "self") reveal that they are composite formations, constructions, fabrications:

"Void is the world...because it is devoid of a self and anything belonging to a self" -- suññam attena vā attaniyena vā (S. XXXV, 85).

This is also said of the Five Aggregates clung to as Self (khandha) in the same text. See also MN 43, MN 106.

The ultimate truth is liberating, but we fear it.

In CNidd. (quoted in The Path of Purification, Vis.M. XXI, 55), it is said:

"Eye, ear, nose, tongue, body, mind, visual objects, auditory objects... mind-objects, visual consciousness, auditory consciousness... mind-consciousness, corporeality... consciousness, and so on -- are devoid of self and anything belonging to a self; void of permanence and anything lasting, eternal, or immutable... They [all phenomena clung to as self] are impersonal: without a core of permanence, a core of satisfaction [the ability to yield fulfillment], or a core of self."
 

In MN 121 the voiding of the mind of the cankers (corruptions), in the attainment of full enlightenment (arhatship), is regarded as the "fully purified and incomparably highest (concept of) emptiness (voidness)."

See Sn.v.1119; MN 121; MN 122 (Wheel #87 at BPS.lk); Pts.M. II: Suñña-kathā; Vis.M. XXI, 53ff.

Explanation

There are progressive stages of awakening.

In Buddhism there are two general kinds of meditation -- serenity (samatha) and insight (vipassana).

One is an emptying of the mind for its purification, serenity that leads to the progressive meditative absorptions (zens, jhanas, dhyanas, seons, channas, chans).

One attains this purified state of consciousness, emerges, and applies a systematic contemplation of the phenomena called the "Five Aggregates clung to as Self":

1. form
2. feelings
3. perceptions
4. formations
5. consciousness.

Both types of meditation require mindfulness, in general for gaining peace of mind, serenity, and purifying-concentration (samadhi) and in specific for engaging in systematic insight-meditation (vipassana) to achieve the progressive stages of enlightenment as one glimpses the supermundane paths and fruitions (magga-phala) and nirvana (ultimate peace, the complete end of suffering).