"In vivo" is latin for ‘in the living’. Therefore, it refers to studies and experiments that occur in an organism that is living, like people, animals, or plants.

"In vitro" is latin for ‘in glass’, which is where experiments outside of a living organism were traditionally performed. These studies are performed in controlled environments like a petri dish which is often made of glass (and more recently, plastic)

Both types of experiments are necessary in science and offer different benefits and drawbacks. In vitro experiments are easier to control, offer more detailed analyses, and can be performed at higher numbers than in vivo experiments, but they cannot yet replicate the complex conditions and give results with as much biological relevance as a whole living organism.

"In situ" is latin for 'on site' or 'in position', and is used to describe events that take place locally or in place for different experimental contexts. For example, examining cells or tissues within a whole organ that is intact and under circulation can be considered an in situ investigation even if . (Opposite is ex situ, referring to something removed from its original position or location)

"In silico" is pseudo-latin for 'in silicon', referring to silicon in computer chips. These studies are experiments performed on computer or via computer simulation.