Virama

Virama[] is a Sanskrit phonological concept to suppress the inherent vowel that otherwise occurs with every consonant letter, commonly used as a generic term for a codepoint in Unicode, representing either

halanta, hasanta or explicit virāma, a diacritic in many Brahmic scripts, including the Devanagari and Bengali scripts, or
saṃyuktākṣara or implicit virama, a conjunct consonant or ligature.

Unicode schemes of scripts writing Mainland Southeast Asia languages, such as that of Burmese script and of Tibetan script, generally do not group the two functions together.

Names

The name is Sanskrit for "cessation, termination, end". As a Sanskrit word, it is used in place of several language-specific terms, such as:

Name in English books	Language	In native language	Form	Notes
halant	Hindi		्
halanta	Punjabi		੍
halanta	Marathi		्
halanta	Nepali		्
halanta	Odia		୍
halanta	Gujarati		્
hosonto	Bengali		্
hosonto	Assamese	/	্
hosonto	Sylheti		◌ ꠆
pollu	Telugu		్
pulli	Tamil		்
chandrakkala	Malayalam	/	്	Unlike other virama diacritics, it is pronounced word-finally.
ardhakshara chihne	Kannada	/	್
hal kirima	Sinhalese		්
a that	Burmese		်	lit. "nonexistence"
viream	Khmer		៑
toandeakheat	Khmer		៍
karan, thanthakhat	Thai	/	◌์	Thanthakhat is the name of the diacritic, while karan refers to the character that was marked. These two terms are often used interchangeably. It is used to mark as silent vowels or consonants that were originally pronounced, but have become silenced in Thai pronunciation. This diacritic is sometimes used in loanwords from European languages to mark final consonants in consonant clusters.
pinthu	Thai		◌ฺ	Pinthu is akin to Sanskrit bindu, and means "point" or "dot". It is used to mark a syllable as closed, and it is only used in Thai script when writing Pali or Sanskrit.
nikkhahit	Thai	นฤคหิต / นิคหิต	◌ํ	Nikkhahit represents what was originally anusvāra in Sanskrit. Like pinthu, it is also only used when writing Pali or Sanskrit in Thai script. It marks a syllable as nasalized, realized in Thai as a nasal closed consonant following the vowel.
rahaam	Northern Thai		◌᩺
rahaam	Tai Khün		◌᩼
rahaam	Tai Lue		◌᩼
wirama	Kawi		◌?
pangkon	Javanese		◌꧀
adeg-adeg	Balinese		◌᭄
pangolat	Mandailing		◌᯲
pangolat	Pakpak		◌᯲
pangolat	Toba		◌᯲
penengen	Karo		◌᯳
panongonan	Simalungun		◌᯳
pamaeh	Sundanese		◌᮪
bunuhan	Rejang		꥓
sukun	Dhivehi		ް◌	Derives from Arabic "sukun"
Srog med	Tibetan	Srog med	྄	Only used when transcribing Sanskrit

Usage

In Devanagari and many other Indic scripts, a virama is used to cancel the inherent vowel of a consonant letter and represent a consonant without a vowel, a "dead" consonant. For example, in Devanagari,

क is a consonant letter, ka,
् is a virāma; therefore,
क् represents a dead consonant k.

If this k क् is further followed by another consonant letter, for example, ṣa ष, the result might look like क्‌ष, which represents kṣa as ka + virāma + ṣa. In this case, two elements k क् and ṣa ष are simply placed one by one, side by side. Alternatively, kṣa can be also written as a ligature क्ष, which is actually the preferred form.
Generally, when a dead consonant letter C₁ and another consonant letter C₂ are conjoined, the result may be:

A fully conjoined ligature of C₁+C₂;
Half-conjoined—
*C₁-conjoining: a modified form of C₁ attached to the original form of C₂
*C₂-conjoining: a modified form of C₂ attached to the full form of C₁; or
Non-ligated: full forms of C₁ and C₂ with a visible virama.

If the result is fully or half-conjoined, the virama which made C₁ dead becomes invisible, logically existing only in a character encoding scheme such as ISCII or Unicode. If the result is not ligated, a virama is visible, attached to C₁, actually written.
Basically, those differences are only glyph variants, and the three forms are semantically identical. Although there may be a preferred form for a given consonant cluster in each language and some scripts do not have some kind of ligatures or half forms at all, it is generally acceptable to use a nonligature form instead of a ligature form even when the latter is preferred if the font does not have a glyph for the ligature. In some other cases, whether to use a ligature or not is just a matter of taste.
The virāma in the sequence C₁ + virāma + C₂ may thus work as an invisible control character to ligate C₁ and C₂ in Unicode. For example,

ka क + virāma + ṣa ष = kṣa क्ष

is a fully conjoined ligature. It is also possible that the virāma does not ligate C₁ and C₂, leaving the full forms of C₁ and C₂ as they are:

ka क + virama + ṣa ष = kṣa क्‌ष

is an example of such a non-ligated form.
The sequences ङ्क ङ्ख ङ्ग ङ्घ, in common Sanskrit orthography, should be written as conjuncts.

End of word

The inherent vowel is not always pronounced, in particular at the end of a word. No virāma is used for vowel suppression in such cases. Instead, the orthography is based on Sanskrit where all inherent vowels are pronounced, and leaves to the reader of modern languages to delete the schwa when appropriate.