In vitro (Latin for (with)in glass) means within a test tube, or just outside a living organism (a cell). An example is In Vitro Fertilization. The opposite of in vitro is in vivo, within a cell. As many experiments that deal with molecular biology are conducted outside of cells, and the conditions do not necessarily represent the conditions inside the cell, results are often annotated with in vivo or in vitro.
... households out of which 22.2% have children under the age of 18 living with them, 55.9% are married couples living together, 8.2% have a female householder with no husband ...