The Electronic Discovery Reference Model (EDRM) defines a custodian as a “[p]erson having administrative control of a document or electronic file; for example, the data custodian of an email is the owner of the mailbox which contains the message.”
In non-legalese, a custodian is a witness (or potential witness) with control of relevant evidence. For instance, assume in a products liability case an employee authored the “smoking gun” research document and saved the document on his or her computer. The employee is the “custodian” of the document because he or she has control of it. Ultimately, the document would be collected from the employee during the discovery phase of litigation and, if it is not privileged, provided (“produced”) to the opponent.
Often, in the early stages of a legal matter, ESI custodians are interviewed by attorneys so the attorneys may better understand what the custodian knows about the legal matter and identify any relevant documents or ESI (electronically stored information) possessed by the custodian. Once the relevant documents are identified, they are preserved for potential use in the litigation.