A glottal stop (ʔ) is a kind of consonant or part of a consonant found in many languages, produced by a complete closure of the vocal cords, blocking the glottis. The glottal stop may be a distinct segment in a language (a phoneme, as in Hawaiian /ʔ/) or it may be an alternate form of a phoneme, such as /t/ in English. The glottal stop, or closure, may briefly halt the airflow from the lungs, or it may combine with other articulatory movements to form ejective or implosive sounds, which make use of a glottalic airstream mechanism.
The [t] sound in English may partially or completely alternate with a glottal stop for many speakers, such as users of Cockney and Estuary English in London and the south-east of England; in other words, [t], [ʔ] and a partially glottalised [t] are allophones (phonetic variants) which represent a single phoneme, /t/, and which one surfaces depends on various factors such as level of formality.
The apostrophe is often used to represent the glottal stop in words taken from languages such as Arabic that have it as a phoneme. This includes non-standard English: "li'l", when referring to Cockney (as opposed to American Southerners, who simply omit the [t] sound), involves partially or completely replacing the [t] of little with a glottal stop.
Ejectives and implosives
Ejective and implosive sounds are found in many languages as either phonemes (as in Korean) or allophones. Ejectives such as [p' t' k'] involve a glottal closure without airflow from the lungs and movement of the glottis upward; this subjects air above the glottis to increased pressure while a closure in the mouth is maintained. When this closure is released, for example by moving the lips apart, the air is suddenly 'ejected' outwards. Given that the glottis must remain closed throughout, voiced ejectives are not phonetically possible. Implosives involve the opposite configuration, with the glottis lowering slightly to reduce pressure, though this time some air can escape through the glottis and potentially allow voicing, since it narrows rather than closes completely. This leads to an inward movement of air as the other closure is released, producing [ɓ ɗ ɠ] or their voiceless counterparts. Some languages of West Africa allow implosives as phonemes; others, including English, may allow them as allophones. Emphatic pronunciations in English, for instance, e.g. [b']illions of pounds! may surface as implosives, and ejectives may occur in some final positions, e.g. [keɪk'] cake, through the combination of a sound such as [k] with a glottal stop.