Refers to the anatomical chest region. More formal and technical than 胸 (むね), often used in medical or descriptive contexts.
The doctor examined the patient's chest with a stethoscope.
Let's take a chest X-ray.
The doctor instructed that a chest X-ray be taken.
He felt pain in his chest and went to the hospital.
I had my chest examined during the regular checkup.
胸部 refers to the chest/thorax region, whereas 腹部 refers to the abdomen just below it, so the two are easily mixed up in anatomical descriptions.
Compound of 胸 (kyō, 'chest') and 部 (bu, 'part, section'), forming a technical term for the thoracic region.