In phonetics, the replacement of a consonant by a glottal stop. In English, specifically the replacement of T. This has spread from the Cockney accent of London into general southern English speech, and is very common in the form known as Estuary English, which is becoming the norm in much of the country.
The glottal stop is the catch in the exclamation oh-oh. In Cockney, [t] was replaced by a glottal stop [?] both intervocalically ('water' pronounced [wO:?@] or [wO:?a] - see SAMPA for the other symbols), and at the end of a syllable ('cut' pronounced [ka?]).
In wider British English this sound first appeared before another consonant: 'not really' = [nQ? ri:li]. Latterly it has crept into all postvocalic positions, for some speakers, and increasingly in all social classes.
For many people who take it upon themself to judge 'good' and 'bad' speech, the glottal stop is perhaps perceived as the ugliest feature of the Estuary accent.
This process has been called glottalling, in contrast to the more widespread term glottalization, which refers to the addition of a secondary glottal articulation to another.
In practice the term is confined to the phenomenon of modern English; but it could be applied to similar changes in other world languages, for example Hawaiian: the glottal stop occurs in the name Hawai'i, and derives from an earlier K, which can still be seen in other Polynesian languages, e.g. the Maori equivalent Hawaiki.