Repetition is a noun for the act of repeating, in other words, saying or doing something someone else has already done or something you have done before. It can be a negative thing or a positive. Some art forms are learned by repetition, while speeches generally should not have a lot of repetition in them.
The term can be used in many different fields where the meaning changes slightly, but it always has to do with something happening again in some form.
The adjective form is repetitional.
Repetitiveness is the noun form of the adjective repetitive, which is used to describe something or someone as having the attribute of repetition. This term is usually negative, as in something repeats without need or becomes very tiresome in its repetition.
The adjective form is repetitively.
In short, repetition is the act of repeating and repetitiveness is the quality of repeating too many times.
Examples
The programs are based on research showing that spaced repetition, or repeated exposure to information at planned intervals, is the most powerful way to fix knowledge in one’s memory. [The Wall Street Journal]
Some of the leaks have been dealt with to avoid a repetition of such incidents, the International Atomic Energy Agency (IAEA) said. Measures have included filling and covering of gaps, recovery of contaminated soil and treating surfaces to prevent rainwater leakages. [Reuters]
Even though his industry’s fee base would then shrink 20 per cent, he says, he is undaunted because while accountants love the cash that income tax return preparation generates, they hate the repetitiveness. [Sydney Morning Herald]