Dictionary.com states that "thorax" indicates the part of the torso in humans or in higher vertebrates that is situated between the abdomen and the neck. The thorax, or chest, includes the cavity where the lungs and heart are located. The ribs, sternum and some vertebrae that surround the cavity.
With respect to insects, Dictionary.com advises the word is used to indicate the part of the insect's body between its abdomen and its head.
The Online Etymology Dictionary states that the word's first recorded usage in English occurred in the late 14th century. The word came to English from the Latin "thorax," which refers to the breast, breastplate or chest. The Latin word comes from a Greek word of unknown origin with identical spelling and meaning.