Oncogene: a gene whose regular function is to promote the division of a cell. When the expression of this gene is increased, the cell divides much more than it should, which can lead to cancer.
A useful analogy is that of a car. When the expression of an oncogene is disrupted, it can be like putting your foot down on the accelerator.
Many drugs were designed targeting these oncogenes to prevent their unbridled expression.
The word comes from the Greek root onkos meaning “mass” or “tumour”. The study of cancer is indeed called “oncology”.