In vivo (Latin for (with)in the living) means within a living organism (a cell). For example, biochemical reactions can take place in vivo. The opposite is in vitro, something that is or happens in an artificial environment (for example, in vitro fertilization). 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.
