Retardation is often used to describe problems in human mental development that result in a lack of intelligence or other kinds of physical or learning delays.
Retardation can also indicate when something is kept from growing or developing fully. A lack of sunlight might cause retardation of your tomato plants, for example, stunting their growth. You can use retardation to describe other kinds of slowness or delays too, including simply a decrease in speed. In fact, the Latin root word, retardare, means "to make slow, delay, keep back, or hinder."