Fortition, also known as strengthening, is a consonantal change that increases the degree of stricture. It's the opposite of the more common lenition. For example, a fricative or an approximant may become a stop (i.e. [v] becomes [b] or [r] becomes [d]). Although not as typical of sound change as lenition, fortition may occur in prominent positions, such as at the beginning of a word or stressed syllable; as an effect of reducing markedness; or due to morphological leveling.
The extremely common approximant sound [j] is sometimes subject to fortition; since it is a semivowel, almost any change to the sound other than simple deletion would constitute fortition. It has changed into the voiced fricative [?] in a number of indigenous languages of the Arctic, such as the Eskimo-Aleut languages and Ket, and also in some varieties of Spanish. In the Southern Ryukyuan language Yonaguni, it has changed word-initially into [d]. Via a voiceless palatal approximant, it has turned in some Germanic languages into [ç], the voiceless equivalent of [?] and also cross-linguistically rare though less so than [?]. Another change turned [j] to an affricate [d?] during the development of the Romance languages.
Fortition of the cross-linguistically rare interdental fricatives [?] and [ð] to the almost universal corresponding stops [t] and [d] is relatively common. This has occurred in most continental Germanic languages and several English dialects, several Uralic languages, and a few Semitic languages, among others. This has the result of reducing the markedness of the sounds [?] and [ð].
Fortition also frequently occurs with voiceless versions of the common lateral approximant [l], usually sourced from combinations of [l] with a voiceless obstruent. The product is a voiceless alveolar lateral fricative [?].
In the Cushitic language Iraqw, *d has lenited to /r/ between vowels, but *r has undergone fortition to /d/ word initially. In Friulan ? >> d : yoyba, jobia > dobia, doba ; gel (tosc. giallo) > dal ; giovane > doven ; giugno > dun 
In addition to language-internal development, fortition can also occur when a language acquires loanwords. Goidelic languages frequently display fortition in loanwords as most initial fricatives (except for [s?], [?] and [f]) are disallowed in the citation form of Goidelic words. Thus initial fricatives of loanwords are strengthened to the corresponding unlenited variant or the nearest equivalent if the fricative is not part of the phoneme inventory.
|/v/||-> /p/||Scots vervane, werwane 'vervain' -> bearbhain /pav/|
|/?/||-> /k?/||Scots quhel 'wheel' -> cuidheall /k?uj?l/|
|/w/||-> /p/||Middle English wall -> balla /pal/|
|/f/||-> /p/||Latin fundus -> bonn /p?un/ (foundation)|
|/?/||-> /t/||Norse þr?ll -> tràill /ta:?/ (slave)|
|/h/||-> /t/||Scots hogsheid 'hogshead' -> tocasaid /t?k?st?/|
|/j/||-> /k?/||English yawl -> geòla /k:l/|
Post-nasal fortition is very common in Bantu languages. For example, Swahili l and r become d after a nasal prefix, and w becomes b; voiceless stops become aspirated. In Shambala, l and r become d, and h and gh [?] become p and g as well. In Bukusu, v [?] and w become b, y becomes j [d?], and l, r become d. In other languages, voiceless fricatives f, s, hl become affricates pf, ts, tl; see for example Xhosa. This is similar to the epenthetic stop in words like dance (['dæns ~ 'dænts]) in many dialects of English, which effectively is fortition of fricative [s] to affricate [ts].