Gradualism is a biological concept that refers to the type of change that characterizes biological evolution. Note, it does not refer to the rate of change.
Gradualism holds that evolution occurs through the accumulation of slight modifications over a period of generations. This is in contrast to saltationism, which holds that evolution occurs through large changes, possibly in a single generation.
Gradualism holds that every individual is the same species as its parents, and that there is no clear line of demarcation between the old species and the new species. It holds that the species is not a fixed type, and that it is the population, not the individual, that evolves.