In phonology, tenseness or tensing is, most broadly, the pronunciation of a sound with greater muscular effort or constriction than is typical. More specifically, tenseness is the pronunciation of a vowel with less centralization (i.e. either more fronting or more backing), longer duration, and narrower mouth width (with the tongue being perhaps more raised) compared with another vowel. The opposite quality to tenseness is known as laxness or laxing: the pronunciation of a vowel with relatively more centralization, shorter duration, and more widening (perhaps even lowering).

Contrasts between two vowels on the basis of tenseness, and even phonemic contrasts, are common in many languages, including English. For example, in most English dialects, beet and bit are contrasted by the vowel sound being tense in the first word but not the second; i.e., (as in beet) is the tense counterpart to the lax (as in bit); the same is true of (as in kook) versus (as in cook). Unlike most distinctive features, the feature [tense] can be interpreted only relatively, often with a perception of greater tension or pressure in the mouth, which, in a language like English, contrasts between two corresponding vowel types: a tense vowel and a lax vowel. An example in Vietnamese is the letters ă and â representing lax vowels, and the letters a and ơ representing the corresponding tense vowels. Some languages like Spanish are often considered as having only tense vowels, but since the quality of tenseness is not a phonemic feature in this language, it cannot be applied to describe its vowels in any meaningful way. The term has also occasionally been used to describe contrasts in consonants.