The flip side of in vitro. It means, unsurprisingly, 'in living' or 'in life' - that is in a living organism or system. Thus, a lab experiment is in vitro while breathing is in vivo.

As a point of philosophical interest, the two situations can be very different. Enzyme kinetics, for example, is often carried out at very low concentrations relative to in vivo conditions. This doesn't mean thet the experiments are 100% wrong, but it is important to remember that when making assumptions. To properly do 'physiological' biochemistry or molecular biology in the lab would be both ruinously expensive (even cheap enzymes can be 20 quid for 10ml - that's as expensive as drugs!) and exceedingly difficult.

See: in silico