Yeísmo (Spanish pronunciation: [e'izmo]) (literally "Y-ism") is a distinctive feature of certain languages, many dialects of the Spanish language in particular. This feature is characterized by the loss of the traditional palatal lateral approximant phoneme [?] (written ⟨ll⟩) and its merger into the phoneme [?] (written ⟨y⟩), usually realized as a palatal approximant or affricate. It is an example of delateralization.
In other words, ⟨ll⟩ and ⟨y⟩ represent the same sound [?] when yeísmo is present. The term yeísmo comes from the Spanish name for the letter ⟨y⟩ (ye). Now, over 90% of Spanish dialects exhibit this phonemic merger. Similar mergers exist in other languages, such as French, Italian, Hungarian, Catalan, Basque, Portuguese or Galician, with different social considerations.
Most dialects that merge the two sounds represented by ⟨ll⟩ and ⟨y⟩ realize the remaining sound as a voiced palatal fricative [?] , which is similar to the ⟨y⟩ in English your, but with stronger friction. However, it sometimes becomes a voiced palatal affricate  , sounding somewhat like ⟨j⟩ in English jar, especially when appearing after /n/ or /l/ or at the beginning of a word. For example, relleno is pronounced [re'?eno] and conllevar is pronounced [ko?e'?a?] or [ko?d?e'?a?].
The sound itself may have originated in Argentina and Uruguay as an influence of local Amerindian languages on the colonial pronunciation of the Spanish language typical of the area's inhabitants of that time, a pronunciation that persisted after the mass immigration of post-colonial Italians and Spaniards into the region which otherwise transformed the demographics and affected aspects of the Spanish language there, including most noticeably intonation. Prior to this post-colonial mass immigration wave, as most other South American countries, their populations were similarly composed of a mestizo majority (those of mixed Spaniard and Amerindian ancestry). In Buenos Aires the sound has recently been devoiced to (sheísmo) among younger speakers.
In the Andean Sierra region of Ecuador, by contrast, where the Amerindian-origin pronunciation of /ll/ as also survives among the majority population of colonial-descended mestizos, the sibilant has not merged as in Argentina and Uruguay, and so a distinction is also maintained but with ⟨ll⟩ representing the phoneme (rather than the original Spanish sound) and ⟨y⟩ representing the phoneme . In the orthography of several Ecuadorian dialects of Quichua, under the influence of the orthography of Ecuadorian Andean Spanish, the grapheme /ll/ is also used to represent the sound. Thus, the Andean Ecuadorian town of Pallatanga, in Chimborazo Province, is correctly pronounced as [pa?atanka] in Quichua, where ⟨ll⟩ represents in the orthography of Ecuadorian Andean Spanish and Ecuadorian Quichua dialects, not as [pa?atanga] as it is pronounced by Coastal Ecuadorians and other yeísta Spanish-speakers.
Rioplatense does not, however, use the sibilant sound for word-initial /i?/ (spelt hi- + vowel). Therefore hierro ['jero] is distinct from yerro ['?ero]. These two words are merged in most other varieties of Spanish.
The same shift from to to (to modern ) historically occurred in the development of Old Spanish; this accounts for such pairings as Spanish mujer vs Portuguese mulher, ojo vs olho, hija vs filha and so on.
Currently, the highlands of Colombia are shifting to yeísmo with older people being the only keeping the distinction, which is completely lost in people born in the 1980s onwards.
Yeísmo has begun appearing in the speech of Ecuador's middle and upper classes.
The retention of a distinction between /?/ and /?/ is more common in areas where Spanish coexists with other languages, either with Amerindian languages, such as Aymara, Quechua, and Guaraní, which, with the exception of Guaraní, themselves possess the phoneme /?/, or in Spain itself in areas with linguistic contact with Catalan and Basque.
In Spain, most of the northern half of the country and several areas in the south used to retain the distinction, but yeísmo has spread throughout the country, and the distinction is now lost in most of Spain, particularly outside areas with linguistic contact with Catalan and Basque.
The relatively low frequency of both /?/ and /?/ makes confusion unlikely. However, orthographic mistakes are common (for example, writing llendo instead of yendo). A notable case is the name of the island of Mallorca: since Mallorcans tend to pronounce intervocalic /?/ as /j/, central Catalan scribes assumed the authentic (and correct) name Maiorca was another case of this and hypercorrected it to Mallorca. This new form ended up becoming the usual pronunciation, even for native Mallorcans.