Human embryonic stem cell: Also known as a human pluripotent stem cell, one of the "cells that are self-replicating, are derived from human embryos or human fetal tissue, and are known to develop into cells and tissues of the three primary germ layers. Although human pluripotent stem cells may be derived from embryos or fetal tissue, such stem cells are not themselves embryos." (From the National Institutes of Health Guidelines for Research Using Human Pluripotent Stem Cells.)
"Self-replicating" means the cell can divide and to form cells indistinguishable from it. The "three primary germ layers" -- called the ectoderm, mesoderm, and endoderm -- are the primary layers of cells in the embryo from which all tissues and organs develop.