lumbocostal triangle of diaphragm (vertebrocostal trigone or triangle, Bochdalek gap)

A triangular area in the diaphragm between its lumbar & costal parts & superior to the lateral arcuate ligament that is devoid of muscle fibers. - covered by pleura superiorly & peritoneum inferiorly - a closure defect of the fetal pleuroperitoneal hiatus- if extensive, results in the most common site of diaphragmatic hernia of abdominal viscera

References

Stedman's Medical Dictionary 27th ed, Williams & Wilkins, Baltimore, 1999.