Fortition (Latin fortis "strong") is the opposite of lenition: a sound change or process by which a consonant becomes phonetically stronger — longer, more constricted, more obstruent. Typical fortition trajectories run glide → fricative → affricate → stop, or move a sound from a sonorant class toward an obstruent one. Fortition is markedly less common cross-linguistically than lenition; languages much more readily weaken sounds than strengthen them.
Fortition tends to occur in prominent positions where speakers naturally hyperarticulate: word-initial, in onsets of stressed syllables, after a pause, or after morphological boundaries. It can also serve to reduce markedness (strengthening a sound that the language treats as suspiciously vowel-like) or to maintain a contrast that lenition would erase. Hayes (2009) describes fortition as much sparser than lenition because the principle of articulatory ease pushes systems toward weaker, not stronger, articulations.
The most frequent type involves the strengthening of palatal /j/ or labial /w/ glides, which are barely consonantal. The palatal glide /j/ has hardened to /ʝ/, /ɟ/, or /dʒ/ in many languages. In Spanish, the yeísmo and rehilamiento developments turned earlier /ʎ/ and /j/ into stronger fricatives or affricates in some dialects (Argentine Spanish yo /ʃo/ or /ʒo/). Yiddish shows initial /j/ strengthening into the affricate area in some lexical items. Several English varieties show comparable hardening of /j/ to /dʒ/ in onset clusters (tube /tjuːb/ → /tʃuːb/, often called yod-coalescence), technically a fortition of the glide combined with assimilation to the preceding stop.
Word-initial position commonly attracts fortition. Latin word-initial /j/ became Spanish /dʒ/ then /x/ (iam > ya, iūnius > junio). Several Germanic languages strengthened initial /w/ to /v/ or /ɡʷ/ in some contexts. Final-obstruent devoicing in German, Dutch, Russian, and Polish is sometimes classed as fortition, though many phonologists analyse it differently.
When a consonant doubles (geminates), it is articulated longer and often with greater closure. Italian fatto "done" with a geminate /tː/ is articulatorily stronger than the singleton /t/ in fato. This kind of length-driven strengthening is sometimes treated as fortition, though others reserve the term for changes in manner rather than duration.
Fortition has minimal direct application in ELT, since it is rare in modern English and not a productive process learners need to acquire. Awareness of yod-coalescence (tube, Tuesday, educate with /tʃ/ or /dʒ/) helps learners decode connected speech in some accents, since the strengthened affricate replaces what dictionaries list as /tj/ or /dj/. Beyond this single area, fortition belongs in the historical-phonology background, paired conceptually with lenition to give a balanced picture of how consonant systems change.