Yeísmo (Spanish pronunciation: [ɟʝeˈizmo]) is a distinctive feature of many dialects of the Spanish language, which consists of the loss of the traditional palatal lateral approximant phoneme /ʎ/ (written ⟨ll⟩) and its merger into the phoneme /ʝ/ (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 /ʝ/. The term yeísmo comes from the Spanish name of the letter ⟨y⟩ (ye). Now, over 90% of Spanish dialects exhibit this phonemic merger.  Similar mergers exist in other languages, such as 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 [ʝ], which is similar to the ⟨y⟩ in English your, but it sometimes sounds like ⟨j⟩ in English jar, especially after /n/ or /l/ or at the beginning of a word. For example, relleno is pronounced [reˈʝeno] and conllevar is pronounced [koɲɟ͡ʝeˈβaɾ] or [koɲdʒeˈβaɾ].
In most of Argentina and Uruguay, the merged sound is pronounced as a sibilant [ʒ]; this is referred to as zheísmo. In Buenos Aires, the sound [ʒ] has recently been devoiced to [ʃ] (sheísmo) among younger speakers.
Note that the same shift from [ʎ] to [ʒ] to [ʃ] (to modern [x]) 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.
Extent of yeísmo and lleísmo
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 1980's onwards.
The distinction between /ʝ/ and /ʎ/ remains in the Philippines, Ecuadoran highlands, Andean Peru, Paraguay, Bolivia, and the northeastern portions of Argentina that border with Paraguay.  The distinction is more common in areas with a common bilingualism with indigenous languages, such as Aymara, Quechua, and Guaraní. 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.
- haya ("beech tree" / "that there be") ~ halla ("s/he finds")
- cayó ("s/he fell") ~ calló ("s/he became silent")
- hoya ("pit, hole") ~ olla ("pot")
- baya ("berry") / vaya ("that he go") ~ valla ("fence")
The relatively low frequency of both /ʝ/ and /ʎ/ makes confusion unlikely. However, orthographic mistakes are common (for example, writing llendo instead of yendo). A similar effect took place in the local name of the island of Majorca: Mallorca is a continental Catalan hypercorrection of the earlier Maiorca.
Similar phenomena occurred in other languages.
- In French, /ʎ/ turned into /j/, but the spelling ll was preserved, hence briller [bʁije].
- In Hungarian, /ʎ/ turned into /j/, but the spelling ly was preserved, hence lyuk [juk].
- In Swedish, /lj/ turned into /j/, but the spelling lj was preserved, hence ljus [jʉːs].
- In Portuguese, yeísmo is observed mainly among speakers of the Caipira dialect (Brazil), who pronounce both /ʎ/ (spelled ⟨lh⟩) and /j/ (spelled ⟨i⟩) as [j]. Hence for those speakers teia "web" and telha "roof tile" are both pronounced as [teja], while in other dialects the later is [teʎa].
- History of the Spanish language
- List of phonetics topics
- Phonological history of Spanish coronal fricatives (distinción, seseo and ceceo)
- "La "i griega" se llamará "ye"" Cuba Debate. 2010-11-05. Retrieved 25 November 2010.
- Coloma (2011), p. 103.
- Álvarez Menendez (2005), p. 104.
- Schwegler, Kempff & Ameal-Guerra (2009), p. 399.
- Travis (2009), p. 76.
- Martínez-Celdrán, Fernández-Planas & Carrera-Sabaté (2003), p. 258.
- Lipski (1994), p. 170.
- Coloma (2011), p. 95.
- Lapesa, Rafael. "El español de América" (in Spanish). Cultural Antonio de Nebrija.
- Coloma (2011), pp. 110–111.
- Diccionari català-valencià-balear, Institut d'Estudis Catalans (look up the term "Mallorca")
- Álvarez Menéndez, Alfredo I (2005), Hablar en español: la cortesía verbal, la pronunciación estándar del español, las formas de expresión oral, Universidad de Oviedo
- Coloma, German (2011), "Valoración socioeconómica de los rasgos fonéticos dialectales de la lengua española.", Lexis, 35 (1): 91–118
- Lipski, John (1994), Latin American Spanish, New York: Longman Publishing
- Martínez-Celdrán, Eugenio; Fernández-Planas, Ana Ma.; Carrera-Sabaté, Josefina (2003), "Castilian Spanish", Journal of the International Phonetic Association, 33 (2): 255–259, doi:10.1017/S0025100303001373
- Navarro, Tomás (1964), "Nuevos datos sobre el yeísmo en España" (PDF), Thesavrvs: Boletín del Instituto Caro y Cuervo, 19 (1): 1–117
- Torreblanca, Máximo (1974), "Estado actual del lleísmo y de la h aspirada en el noroeste de la provincia de Toledo", Revista de dialectología y tradiciones populares, 30 (1-2): 77–90
- Schwegler, Armin; Kempff, Juergen; Ameal-Guerra, Ana (2009), Fonética y fonología españolas, John Wiley & Sons, ISBN 0470421924
- Travis, Catherine E. (2009), Introducción a la lingüística hispánica, Cambridge University Press
- Pharies, David (2007). A Brief History of the Spanish Language. University of Chicago Press. ISBN 978-0-226-66683-9.