Hypertrophy is defined as the enlargement of an organ or tissue from the increase in the size of its cells.
In the strength and conditioning world, we have an understanding of two types of hypertrophy (although both are interchangeable, as to the outcome of gross muscle hypertrophy). These are names coined Myofibril and Sarcoplasmic hypertrophy.
Myofibril hypertrophy (or functional hypertrophy) aims to increase the activation of satellite cells within the muscle, to initiate gene expression of increasing protein density of contractile proteins within the muscle. This is generally done by exposure of near to maximal load and reduced overall tension time exposure.