in an artificial environment. For example many cancer research experiments are in vitro (in the test tube), using cell cultures (cells grown in the lab); either from established cell lines or from material collected at biopsy/surgery. See also: In-Vivo
Within an artificial medium like a test tube or culture dish, outside and separate from an entire living organism. Example: studies done on cells in vitro means test tube cultures of cells are grown and studied for one or more of their properties.
'In-glass', referring to use of a cell line, microorganisms or biochemically active fraction derived from an organism (e.g. hepatocytes) in laboratory studies of biological activity, or metabolism or toxicity.