secret agent

: kept from knowledge or view : hidden

: marked by the habit of discretion : closemouthed

: working with hidden aims or methods : undercover

: something kept hidden or unexplained : mystery

: something kept from the knowledge of others or shared only confidentially with a few

: a method, formula, or process used in an art or operation and divulged only to those of one's own company or craft : trade secret

: one that acts or exerts power

: something that produces or is capable of producing an effect : an active or efficient cause

: a chemically, physically, or biologically active principle