An affricate is a consonant that begins like a stop (most often an alveovelar, such as [t] or [d]) and that doesn't have a release of its own, but opens directly into a fricative (or, in one language, into a trill).
The English sounds spelt "ch" and "j" (transcribed [tʃ] and [dʒ] (or [tS] and [dZ] in SAMPA)), German and Italian z [ts] and Italian z [dz] are typical affricates. These sounds are fairly common in the world's languages, as are other affricates with similar sounds, such as those in Chinese.
Much less common are e.g. labiodental affricates, such as [pf] in German, or velar affricates, such as [kx] in Setswana (written kg) or High Alemannic Swiss German dialects (depending on the dialect also uvular [qχ]). Worldwide, only a few languages have affricates in these positions, even though the corresponding stop consonants are virtually universal. Also less common are alveolar affricates where the fricative is lateral, such as the [tɬ] sound found in Nahuatl and Totonac. Many Athabaskan languages (such as Navajo) have series of coronal affricates which may be unaspirated, aspirated, or ejective in addition to being alveolar, postalveolar, or lateral, i.e. [ ts ], [ tsʰ], [ ts̕ ], [ tʃ ], [ tʃʰ ], [ tʃ̕ ], [ tɬ ], [ tɬʰ ], and [ tɬ̕ ].
Affricates are often represented by the two sounds they consist of (e.g. [ts, kx]). However, single signs for the affricates may be desirable, in order to stress that they function as unitary speech segments (i.e. as Polish: [ʧ] in czysta 'clean (f.)' vs. [tʃ] in trzysta 'three hundred',
- Klallam: [ʦ] in k’ʷə́nc 'look at me' vs. [ts] in k’ʷə́nts 'he looks at it'.
The difference is that in the sequence of stop and fricative, the stop has a release of its own before the fricative starts.
Affricates and sequences of stop plus fricatives may also be distinguished in languages that don't contrast them, e.g. in English, where [ts], [dz] (as in nuts, nodds) are considered to be sequences of stop plus fricative even though they would be affricates according to the above phonetic definition.
The reason why they're considered to be sequences of stop plus fricative is that beyond mere phonetics, English [ts] and [dz] are analyzed into different morphemes (e.g. nuts is nut + s), whereas the true English affricates [tʃ] and [dʒ] aren't composed, but considered phonemes of English. In order to show this, they can be written with /ʧ,ʤ/ or /t͡ʃ, d͡ʒ/ or /č, ǰ/.
- Montler, Timothy. (2005). [personal communication].