In vitro (Latin: (with)in the glass) refers to the technique of performing a given experiment in a test tube, or, generally, in a controlled environment outside a living organism. In vitro fertilization is a well-known example of this. Many experiments in cellular biology are conducted outside organisms or cells, thus, the conditions and, therefore, results may not correspond to those inside. Consequently, experimental results are often annotated with in vitro or its opposite in vivo as it applies.
from wikipedia