Making multiple copies of a gene or of any sequence of DNA. Repeated copying of a piece of DNA. 

In amplification there is an increase in the number of copies of any particular piece of DNA. Gene amplification plays a role in cancer cells. A tumor cell amplifies, or copies, DNA segments as a result of cell signals and sometimes environmental events. 

Amplification can occur in vivo (in the living individual) or in vitro (literally "in glass", or in a plastic vessel in the laboratory).