The appearance of a frown varies from culture to culture. Although most technical definitions define it as a wrinkling of the brow, in North America it is primarily thought of as an expression of the mouth. In those cases when used iconically, as with an emoticon, it is entirely presented as a curve of the lips facing away from the eyes ('down'). The mouth expression is also commonly referred to in the colloquial English phrase "turn that frown upside down" which indicates changing from sad to happy.
It most commonly expresses sadness, dissatisfaction, anger, worry or pain, although it is also a sign of deep concentration.