NDA is an acronym for Non-Disclosure Agreement. It is also known as a confidential disclosure agreement (CDA) and proprietary information agreement (PIA).
It is a legal contract between at least two parties that outlines confidential materials, knowledge, or information that the parties wish to share with one another for certain purposes, but wish to restrict access to. It is a contract through which the parties agree not to disclose information covered by the agreement. An NDA creates a confidential relationship between the parties to protect any type of confidential and proprietary information or trade secrets. As such, an NDA protects non-public business information. NDAs are commonly signed when two companies, individuals, or other entities (such as partnerships, societies, etc.) are considering doing business and need to understand the processes used in each other's business for the purpose of evaluating the potential business relationship. NDAs can be "mutual", meaning both parties are restricted in their use of the materials provided, or they can restrict the use of material by a single party.
It is also possible for an employee to sign an NDA or NDA-like agreement with an employer. In fact, some employment agreements will include a clause restricting employees' use and dissemination of company-owned "confidential information."
In rare cases, the contract may state that the existence of the NDA itself cannot be disclosed.