Adult stem cells:
Stem cells found in mature tissues (bone marrow, skin, brain, etc.) that can self-renew and give rise to other cell types from their tissue of origin, thereby producing a steady supply of new cells to maintain that tissue throughout life. For example, a hematopoietic (blood) stem cell found in the bone marrow is an adult stem cell that gives rise to red blood cells, white blood cells and platelets.  In general, adult stem cells from one organ do not give rise to cell types from other organs.

An early stage embryo, prior to the time of implantation in a uterus. The blastocyst consists of a microscopic cluster of several hundred undifferentiated cells, from which embryonic stem cells can be derived.

Cord blood stem cells:
Stem cells that are recovered from umbilical cord at the time of birth. These blood stem cells resemble the stem cells found in adult bone marrow. They are thus more like adult stem cells than embryonic stem cells.

The process whereby a stem cell loses its capacity for self-renewal and becomes a mature and specialized cell type - for example a blood cell, nerve cell, pancreatic cell, cardiac muscle cell, etc.

Embryonic stem cells:
Stem cells that are derived from a blastocyst and which (unlike adult stem cells) can give rise to every cell type in every organ of the body.

Induced pluripotent stem cells:
Induced pluripotent stem cells (iPS cells) are cells that began as normal adult cells (for example, a skin cell) and were engineered or 'induced' by scientists to become pluripotent, or able to form all cell types of the body. This process is often called reprogramming.  While iPS cells and embryonic stem cells share many characteristics, they are not identical and researchers are currently exploring the ways in which they differ.

Multipotent stem cells:
Multipotent stem cells are those cells that give rise to a small number of different cell types. They are one of the three classes of stem cells: multipotent, pluripotent and totipotent.

Pluripotent stem cells:
Pluripotent stem cells are those cells that are capable of giving rise to all cell types in the body, but do not give rise to the supporting structures (placenta, amnion, chorion) required for development of an organism. They are one of the three classes of stem cells: multipotent, pluripotent and totipotent.

The ability of a stem cell to divide and give rise to more stem cells with identical properties to the parent cell, thereby allowing the population to be replenished indefinitely.

Somatic cell nuclear transfer (sometimes known as ‘therapeutic cloning'):
A process by which a nucleus from a single cell (for example a skin cell) is transferred into an unfertilized egg, which is then allowed to develop to the blastocyst stage. Embryonic stem cells derived from this blastocyst are genetically identical to the donor of the original nucleus.

Stem cell:
A cell that can divide to form more stem cells as well as giving rise to specialized cells (blood cells, nerve cells, muscle cells, etc.).

Totipotent stem cells:
Totipotent stem cells are those cells that have total potential to develop into an organism.  A fertilized egg is a totipotent stem cell.  They are one of the three classes of stem cells: multipotent, pluripotent and totipotent.