Tense vowel

Definition
Tense is a feature which characterizes vowels that are produced with a tongue body or tongue root configuration involving a greater constriction than that found in their lax counterparts; this greater degree of constriction is frequently accompanied by greater length (Halle &amp; Clements (1983:7)).

Example
In English, [I] in willis lax, while [i] in wheel is tense.

Links

 * Utrecht Lexicon of Linguistics