Definition of ectoderm
1 a
: the outermost of the three primary germ layers of an embryo that is the source of various tissues and structures (such as the epidermis, the nervous system, and the eyes and ears)
b
: a tissue (such as neural tissue) derived from this germ layer
2
: the outer layer of cells of a diploblastic animal (such as a jellyfish or a hydra)
