Hypertrophy is the enlargement of a body part due to increased size of the individual cells of that part. It is important to distinguish between hypertrophy and hyperplasia, which is an increase in body part size due to an increase in the number of cells. What makes cells become larger? In general, an increase in demand on a cell will make it grow in size, if it can. The classic example is the muscle of a trained body builder. As the body builder "pumps up" with training, the individual cells of his muscles get bigger. The result is growth in muscle size.
The muscles do not, however, gain in the number of cells. The reason is that most muscle cells, including heart cells, cannot divide to increase in number. The brain and nerves also have this limitation. This becomes crucial when injury occurs. When heart muscle cells die after a heart attack, or brain cells die after a stroke, the damaged cells cannot be replaced by cell division. Compare this to the liver, which can fairly quickly grow back to normal size by cell division if even half of it is removed. A damaged heart or brain, on the other hand, remains damaged for life.
What governs if a cell can grow in size? An adequate supply of nerves, blood vessels to bring nutrients, and hormonal support are essential. If these three things are present, a cell can grow to meet increased demands. If these are lacking, a cell cannot grow or even maintain normal size. When deprived of these supports, cell will shrink, a process called atrophy.
Is hypertrophy good or bad? It depends. Larger stronger muscles are essential for the athlete. For the heart, the story is different. If the heart is strained by disease, the cells can grow larger to pump harder and compensate. Eventually, however, a hypertrophic heart is likely to fail. Hypertrophy can be genetic and fatal. Asymmetric septal hypertrophy of the heart is an inherited disease which can be deadly.
Most parts of the body, except for muscles, brain, and nerves, can increase both in cell number and cell size. Both can happen at the same time. An enlarged prostate in an aging man, for instance, is often called "prostatic hypertrophy". The enlargement may involve hyperplasia as well as hypertrophy. The two processes are not mutually exclusive in many parts of the body.
Hypertrophy is, therefore, an increase in a body part size when the individual cells grow in size, but not in number.