Development: The process of growth and differentiation. The most important stage of human development occurs before birth, as tissues and organs arise from differentiation of cells in the embryo. This process continues until birth, and interruptions in development result in the most serious types of birth defects, such as anencephaly and spina bifida. The developmental process continues after birth, as an infant or child grows physically, develops basic brain-based abilities such as speech and hand'eye coordination, and learns. Interruptions in any of these processes can result in developmental delay.