non-disclosure agreement       
noun [ C ] uk/ˌnɒn.dɪˈskləʊ.ʒər əˌɡriː.mənt/ us/ˌnɑːn.dɪˈskloʊ.ʒɚ əˌɡriː.mənt/ (abbreviation NDA)
 保密协议
a legal document stating that someone will not tell anyone certain private information, especially about the company they work for
  He has new employees sign a non-disclosure agreement limiting what they can say about his company. 他让新员工签署保密协议来限制他们(对外)透露他公司的情况。 
 He declined to name the investor, citing a non-disclosure agreement.  他引用保密协议来拒绝说出投资人的名字。 
 - More examples
 - Non-disclosure agreements restrict what workers can say about former employers.
 - He said he signed a non-disclosure agreement when he left the military and could not discuss his relationship with JPRA.
 - After non-disclosure agreements were signed, the suitcase was opened and out came a 3-foot-long scale model of a sleek, futuristic-looking jet.