HESC is an abbreviation for human embryonic stem cell - a stem cell derived from a human embryo.
Pre-implantation human embryos
The stem cells are usually taken from early embryos that are 'left over' after a woman gives her eggs to be fertilised in the laboratory and stored for later implantation into her womb or a surrogate womb. Usually more embryos are created than are used, and some of these are given for the creation of stem cell lines as the individual cells in the early clump of embryonic cells have the potential to multiply indefinitely under the correct conditions and to differentiate into different tissues under the right stimuli.
Human embryonic stem cell (hESC) and human embryo research. Shand J, Berg J, Bogue C; Committee for Pediatric Research; Committee on Bioethics. Pediatrics. 2012 Nov;130(5):972-7. doi: 10.1542/peds.2012-2482. Epub 2012 Oct 29..