A computer made by IBM oriented toward scientific work. The 709 had the same basic architecture as the IBM 704 but with many I/O and performance refinements over the 704.
The IBM 709 (like the 704) had 36-bit memory words, 15-bit addresses and instructions with one address. A few index register instructions had the infamous 15-bit decrement field in addition to the 15-bit address.
The IBM 7090 was a transistorised version of the 709.
[Difference between 704 and 709?]