Fortis (from Latin fortis "strong") and lenis (from Latin lenis "weak") are linguistics terms.
In a wide sense, they refer to the opposition of consonants such as p, t vs. b, d. In a narrow sense, fortis refer to consonants such as p, t pronounced with tenseness (more muscular tension) and lenis to consonants such as b, d pronounced without.
History and use of the terms
These terms have already been used in 19th century German Linguistics, in order to describe languages such as southern German where consonants such as b, d are voiceless but nonetheless different from p, t. The terms are only seldomly used in current linguistics.
Opposition of p, t versus b, d
Normally, the opposition of consonants such as p, t vs. b, d is described in terms of voiceless consonants vs. voiced consonants. There are languages where this is indeed the only feature that distinguishes them (e.g. French: IPA [p, t] vs. [b, d]). In many languages, however, the phonetic voice is only one of several features that constitutes this opposition (e.g. English: IPA [pʰ, tʰ] vs. [b, d]). There are even languages where the phonetic voice is not a distinctive feature of these pairs at all (e.g. southern German: IPA [pʰ, tʰ] vs. [b̥, d̥]).
The terms fortis and lenis (in the wide sense) apply to this opposition regardless of whether it's only an opposition of voice or not. Therefore, it allows to speak in the same terms of French, English, or southern German consonants.
Consequently, they refer to a bundle of articulatory features which have different distributions in different languages. Not all of them need to be present in a particular language:
Tenseness
The tenseness is the feature that distinguishes fortis and lenis in the narrow sense: In the articulation of the fortis, more muscular energy is used.
In Korean, a higher fundamental frequency of the following vowels is thought to be a result of increased muscular tension in stop consonant. In Swiss German, no possible acoustical correspondent of the assumed tenseness has been found. Consequently, it is debated whether the Swiss German opposition is really based on different muscular tension, and not on gemination.
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