A leach may refer to any of the following:
1. In metallurgy, leach means to extract specific metals from other substances; usually by converting them into soluble salts in an aqueous solution. For example, gold is an excellent conductor of electricity, making it useful in circuit boards. Thus leaching the gold from circuit boards is a popular process because it allows the precious metal to be reused elsewhere.
The chemicals used in the leaching process are toxic to humans. Protective gloves and a respirator are advised as exposure to bare skin or inhalation of the fumes may cause health issues.
2. When referring to copying or sharing, a leach is a person who copies lots of data, but never returns the favor. For example, someone who downloads videos from a P2P service, but never uses their computer to share the downloaded files or other files with others. Some networks that share files may setup limits to help prevent leaching by only allowing each person to download so much unless they share files themselves.