a continuously dividing population of cells obtained from human or animal tissues

a group of cells that can self-replicate outside the body for an extended period of time

a growing collection of similar cells, all cultured from a stem cell

a mass of cells descended from the original cell, sharing its genetic characteristics

a reservoir of cells derived from a single human embryo

Stem cells which have been cultured under in vitro conditions that allow proliferation without differentiation for months to years. (NIH)

A stem cell line is a family of constantly-dividing cells, the product of a single parent group of stem cells. They are obtained from human or animal tissues and can replicate for long periods of time in vitro ("within glass"; or, commonly, "in the lab", in an artificial environment).