(2) Short for common ISDN application programming interface, an API standard used to access ISDN equipment (equipment that use the integrated services digital network [ISDN] standard for the transfer of data over telephone lines). When an application wants to communicate with an ISDN card it sends a standard series of commands to the card. These commands form the CAPI standard and give developers and users a chance to use a well-defined mechanism for communications over ISDN lines without being forced to adjust to hardware idiosyncrasies.