Refers to the fleshy parts around the mouth. Can be singular or plural depending on context.
彼女は唇にそっとキスをした。
He gently kissed her on the lips.
寒さで唇が紫色になった。
My lips turned purple from the cold.
He bit his lip in frustration.
My lips get dry in winter.
She licked her lips nervously.
I put lipstick on before going out.
His lips turned blue from the cold.