Fortition

Fortition is a consonantal change from a 'weak' sound to a 'strong' one, the opposite of the more common lenition. For example, a fricative or an approximant may become a plosive (i.e. becomes  or  becomes ). An approximant is also considered to be affected by fortition if it becomes a fricative. Voiced plosives or fricatives becoming voiceless can also be considered fortition.

Examples
The extremely common approximant sound palatal approximant is sometimes subject to fortition; being a semivowel, almost any change to the sound save from simple deletion would constitute fortition. It has changed into the very rare voiced fricative voiced palatal 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. Via a voiceless palatal approximant, it has turned in some Germanic languages into voiceless palatal fricative, the voiceless equivalent of and also cross-linguistically rare though less so than. Another change turned to an affricate Voiced postalveolar affricate during the development of the Romance languages, possibly through an intermediate stop Voiced palatal plosive.

Fortition of the cross-linguistically rare interdental fricatives and  to the almost universal corresponding stops  and  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.

Fortition also frequently occurs with voiceless versions of the common lateral approximant alveolar lateral approximant, which are usually source from combinations of with a voiceless obstruent. The product is a voiceless alveolar lateral fricative.

An example of simultaneous fortition and lenition is Spanish, which changed into  medially, but exhibits the opposite change in initial position. This resulted in the corresponding phonemes (written b and v) merging into one.