A recursive algorithm was a programming code block which called itself over and over again, eventually terminating when an end condition was met.
Most Cardassian encryption codes are based on a recursive encryption algorithm. However, a transmission received by Deep Space 9 in 2373 used a completely different coding system. (DS9: "In Purgatory's Shadow")
That same year, Lieutenant Torres ran a recursive search algorithm to retrieve a gap in the memory index of a Druoda series 5 long-range tactical armor unit that had taken over The Doctor. She found that the armor unit had been launched accidentally, and was deliberately ordered to crash on an uninhabited planet by its people. The missile however did not believe the orders were genuine. (VOY: "Warhead")