:Orbicularis oris muscle:

Pronunciation GB: ɔːˌbɪkjəˌlɑːrɪs ˈɔːrɪs, GA: ɔːrˌbɪkjəˌlærɪs

The most important muscle controlling the position of the lips. Its contraction is necessary both for lip closure in bilabial sounds and for lip rounding in rounded vowels and consonants.