Advanced Tongue Root (definition)
When a vowel or consonant is pronounced with the tongue root (the part of the tongue in the pharynx or throat) pulled forward in the mouth, they are said to be Advanced Tongue Root [+ATR] , or Tensed. In Scottish Gaelic, Tenseness is an effect found on sonorants.
In English tense vowels include /i, e, o, u/.
Ladefoged, Peter (2005) A Course in Phonetics. Thomspson Wadworth Publishers.